The best course of action, in this case for you, is to invent something. Throughout history, there has been many very complicated inventions, like computers, cellphones, cars, etc.
Those inventions, even if you know about them, you wouldn't be able to create them from scratch if you never studied them in the present.
On the other hand, some inventions are literally just a very good idea. If you know it's possible, it might take you a few hours or a few years, but you can make it. For example, if you traveled to the VERY distant past, you'd be able to "invent" fire, wheels, and even more advanced objects like boats and bicycles.
In 1317, honestly the biggest thing that comes to mind that wasn't invented yet but is still simple enough that you could figure it out, is the printing press.
It is a fairly complicated machine, but the "how it works" part is simple enough that your character definitely understands it without having done specific studies. So he could probably make it work within months, or years if many events slow him down.
The goal is basically to invent something that would leave a HUGE impact in the future (inventing the printing press one century earlier would definitely change the world). So you find something that would change history a lot, so there is absolutely no way you can be mistaken about "did history change?", but is still simple enough that it is believable a random guy from 2017 would be able to make it from scratch without plans.
Then, if you are in a stable time-loop type of world, something will prevent you inventing it or making it public. Maybe events out of your control keep stopping you from building it (natural disasters, vandalism, etc). Maybe you get killed before completing it and nobody knows what that half-machine in your workshop was supposed to be. Maybe you get kidnapped by a guy who sees the potential of your machine, steals it, and tries to force you to give its secret. Then plot twist, that guy was Gutenberg's grandfather and it took them two generations to figure out how to complete this half-machine they stole from you. The possibilities are endless.
If you succeed at making your printing press, and making it public enough that no reasonable event could wipe out all traces and memories of its existence within a hundred years, you know you are either in a "Alternate timeline" or "you can change the past" situation.
The question is now: How do you know all those things that stopped your invention weren't just bad luck?
The answer is: you don't. You have to rely on math for this.
Imagine a random number generator, which gives you a random number between 0 and 9 every time you ask. The first time it gives you 4. Sounds pretty fair. Then you get a 4 again. Having twice the same number in a row had a 10% chance of happening, this is Lucky. Then you get another 4. Now it was 1/100, what a coincidence! Then another. Then another. Then another. At some point, it sounds like the machine isn't really random. There is still a very small chance it was only luck, but after getting 20 4s in a row, which had a chance of 1/10000000000000000000, you can pretty much say that it's not random. You can never be sure, but you can be pretty confident.
The same goes for your attempts. If a random group of kids break in your workshop and destroy the machine, that's possible. If the village gets flooded and you lose all your work, that's possible. If your house burns down and you have to start over, that's possible. But every time you see a somewhat-reasonable event that happens to ruin or seriously set back your invention, it could be complete luck, but at some point you have to decide "yeah, having all those events in a row is pretty unlikely. I guess it's not just bad luck after all". You won't have 100% certainty, but I think 99.99999% would be enough for you.
This idea also had a great advantage. If you don't succeed at ever going back to your 2017, you will most likely become rich and famous in the 14th century, and be in most history books later on. That's pretty significant.