How would she be immediately perceived by the citizens of the time?
The reason this question is hard to answer is, it depends. Whoever finds this girl first is going to determine her entire future.
Let's start off by setting the scene.
She is here (just south of the solid black line):
None of these are cities. I'm guessing they're mostly farming/pastural communities back in 1700. Small towns. Villages really. The Industrial Revolution didn't begin for another few decades. It, along with steam engines, didn't happen until the mid-18th century, when your girl would be middle-aged.
Aside from agriculture, fabric was likely the primary industry of the time and region.
By 1600 Flemish refugees began weaving cotton cloth in English towns
where cottage spinning and weaving of wool and linen was well
established; however, they were left alone by the guilds who did not
consider cotton a threat...In 1700 and 1721 the British government passed Calico
Acts in order to protect the domestic woollen and linen industries
from the increasing amounts of cotton fabric imported from
The demand for heavier fabric was met by a domestic industry based
around Lancashire that produced fustian, a cloth with flax warp and
cotton weft. Flax was used for the warp because wheel-spun cotton did
not have sufficient strength, but the resulting blend was not as soft
as 100% cotton and was more difficult to sew.
On the eve of the Industrial Revolution, spinning and weaving were
done in households, for domestic consumption and as a cottage industry
under the putting-out system. Occasionally the work was done in the
workshop of a master weaver. Under the putting-out system, home-based
workers produced under contract to merchant sellers, who often
supplied the raw materials. In the off season the women, typically
farmers' wives, did the spinning and the men did the weaving. Using
the spinning wheel, it took anywhere from four to eight spinners to
supply one hand loom weaver. (ref)
What does this girl look like to the locals?
First of all, maybe she doesn't look like a girl at all. Sure, she's wearing a skirt, but it's not the wide hipped flowing skirt of the late Stuart Period. She has no petticoats or elaborate bodice.
In fact, her outfit looks a lot more like this rich gentleman's. Boots and leggings and all.
While no one could mistake her for a boy after a day or two, it's very possible that whoever first encounters her will assume she is a pre-pubescent boy. The higher voice and lack of facial hair would fit right in. As long as her clothes aren't tight and she's closer to 12 than 14, her body wouldn't necessarily mark her as female. This might give her some initial protection and better explain what she's doing in town alone.
Everyone knows she's not local because the town is small enough that the residents all know each other. The most likely explanation is that she is a salesperson traveling from town to town to sell her (his) wares. She could even tell people her father is camping in the woods and sent her to drum up business. Or she could be looking for work.
Ideally, she'll find an adult she can trust who will take her in for a few days until she can get settled. Those first few minutes will make all the difference.
Despite her soft hands, good skin (not much work in the sun), and excellent teeth and health, she will not be mistaken for a noble. Her clothes might be devilishly fine weaving (a t-shirt is not something people of that time period could duplicate, despite the fact that they were skilled with cotton) but they were just not fancy in any way.
The boots, sure, they're very expensive work, but they're probably scuffed up (teenagers!) and maybe she got them as hand-me-downs from her last employer. She can't even afford petticoats or proper sleeves! No way is she from a wealthy family. Or even merchant class.
What could she bring with her?
If she somehow knew this could happen and wanted to bring things ease her transition into early 18th century life, what could she fill a satchel with that would be easy to bring on a field trip?
Lace was all handmade back then and took a very long time to make. You need tiny crochet hooks (for tatting) or a needle and thread and great skill and practice.
Lace, a decorative openwork web, was first developed in Europe during
the sixteenth century. Two distinct types of lace making—needle lace
and bobbin lace—began simultaneously. Needle lace is made with a
single needle and thread, while bobbin lace entails the plaiting of
many threads. Lace thread was typically made from linen, and later
silk or metallic gold threads, followed by cotton in the nineteenth
Lace was always an expensive luxury item because of its painstaking,
time-consuming production....Both men and women wore
lace from its inception to the eighteenth centuries. It was often the
most costly part of dress and reflected the sophisticated tastes of
the aristocracy. Lace adorned women’s and men’s collars and cuffs,
draped women’s shoulders, hands, heads, covered entire gowns, and
decorated furnishings. The excessive sums of money spent on
extravagant laces prompted many rulers to place restrictions on the
wearing and importing of lace from other countries. Sumptuary laws,
however, proved futile and the smuggling of foreign lace was
widespread. Europe’s desire for handmade lace continued unabated until
the end of the eighteenth century. (ref)
If your character buys pretty machine-made lace.—not the cheapest polyester, but decent quality cotton lace, or lace from synthetics that resemble silk and hold up well—she will be well on her way to having her own business.
Even simple lace trim cost a fair bit. Add in some other decorative needlework, and you have an outfit fit for a queen. (Note that the lace trim around her neck, as well as the trim beneath it, are items that one would purchase separately then sew onto the clothing.)
Queen Anne by Closterman 1702. Hanging in Hampton Court Palace.
It was quite normal for salespeople to wander the countryside with goods they acquired from others who went on long distant trips to trade. As long as she is not immediately robbed (and in a small town, that's probably not going to happen...towns with that reputation don't get visits from salespeople and that can ruin their economy...towns often had fairs and other events where trade was important).
It's your story, if you want her to find kind protective people in the first hour of her stay, then she will.
Detail of Irish Crochet Lace work on collar in the Sheelin Antique Irish Lace Museum in village of Bellanaleck, County Fermanagh
While even plastic jewels would fetch some money, she'd be better off bringing good inexpensive gems made out of glass or less expensive rocks or even some cubic zirconias. Not to pass off as diamonds, emeralds, whatever. But as gems in their own right. She doesn't have to lie: whatever she brings will not be something available there and will be valuable.
Paper and pencils.
Paper will not be easy to find and it will not be cheap. A couple spiral notebooks will be very valuable to her. A bunch of pencils will last longer than ballpoints, but both are useful.
Names and addresses of nearby barristers (lawyers).
This one will take some research but it shouldn't be too hard to look up modern-day firms and find one or two that were founded in or by the early 18th century (she might have to go to London for this, but in 20 years, she'll have the means). Then she can do the old time-travel troupe of leaving a message to be delivered in 2019 to her family.