Assuming we have a way to generate the numbers, like Dragongeek's answer (solar radiation), the problem is to make sure no-one stole those information, like pointed out by mjt and SRM. This would violate #8 as a fake traveler could find out by forcing a real traveler to reveal information.
Additional random questions
First I want to address mjt proposal as I don't think it to be viable:
The only way around this I can think of would be asking the time traveler questions so arbitrary that nobody would have thought to brief their fake about.
The problem is that if nobody would have thought about them, then the same can be said for someone that came from a 100 years from the future and didn't thought of researching those things. Anything he could know from his life experience would be too far in the future to be verified, leading to plausible deniability for any fake.
Characteristic of the body
For this reason I think SRM is right to say that we need to focus on some characteristics of the person to check that it's actually them to have came back from the future. The problem is that it's not possible either.
Many of the following require state-like organizations with a lot of funds. This seems to be within what described by #5. It may be implausible for many scenarios, but it is still be a valid threat according to the question.
If you want to fake the concentration of C14 (for bomb pulse dating too as it's always about the concentration of C14) you just need a closed ecological system able to sustain a human and built with the required initial concentration of C14. It may require some time initially to bootstrap the process, but everything you need it a source of carbon with a the ratio of C14/C12 you want. This can be obtained in at least three ways:
- The first is to use the same centrifugal method used to separate uranium-235 from uranium-238 as their mass ratio is 0.987, while the one of C12/C14 is 0.857. As lower ratio implies a greater difference in mass (more far from 1), this method is viable.
- Using the C14 produced by nuclear power plants as they have a higher C14 concentration. If mixed it can lead to the required ratio.
- Using the carbon from fossil fuels as they have no C14. This can be mixed too in case you need to lower the amount of C14 present.
What you need for the fake is someone building this thing before the time traveler arrives (you wouldn't have a fake always ready if you build it afterwards). You can use the plants grown within it to feed someone using a vegan diet and the result is that the person in question will have the required amounts of C14. From https://en.wikipedia.org/wiki/Carbon-14#In_the_human_body :
Since many sources of human food are ultimately derived from terrestrial plants, the relative concentration of carbon-14 in our bodies is nearly identical to the relative concentration in the atmosphere.
Which obviously means that if we fake the concentration of C14 in the atmosphere used by the plants we eat, then our concentration of C14 will follow the one of that atmosphere. You just need to eat from that closed ecological system from birth, after all the intake of carbon of humans comes from their food. If the 2° or 3° way to modify the C14 ratio are used then this is not even that expensive, you just have to use waste products.
Using C14 to date things is usually considered reliable because no one has a closed ecological system (even if they exist) with altered C14 (even if it's easy to do) from which they eat since birth. But it can be done in the same way it's possible to control stock prices. If you are the USA, EU or Russia of the past, you would just had to convince the ones who actually build the closed ecological system in existence to use the materials and CO2 you provide them. If they were at war and that could make them win it, it wouldn't be that far fetched of a scenario.
One method could be to exploit the case in which the person who came back from the future was already alive (by comparing with the person already alive). The problem is that it can't be used to roll out fake travelers as the fake one could just say to be from a more remote future (like 30 years for a 20 yo.).
For the fake you just need someone that is either from a state with a loose identification or that was kept hidden by a state for military purposes (you can't control what your enemy state is up to). In this way you can't verify that they were already alive in our time and you need to apply other techniques, bypassing any control and assurance that those techniques may have.
DNA coding can be replicated too. Overall it's called gene therapy and there are already treatments able to change the DNA of a part of the body, with the change taking place within the body.
Overall if a state wanted to create a fake time traveler using the information of a valid one they could just apply to many people the best treatment known, possibly multiple times to make sure all cells are affected and use the best fake. But even if not fully possible, this would heavily restrict this idea to the present day. Considering the current developments rates, in just 10 years this technique would become rather unreliable.
Numbers for 20 years
If the time traveler wanted to makes sure to be identified even if he get arrested and released after a while, he may want to remember a set of numbers for some of the following years. Maybe he thinks that he will be asked to be identified multiple times or just to be sure in case something happens. The problem is that we can't control or check that they didn't remember additional numbers, so this scenario applies to any method used to prove that a certain person is a time traveler.
The problem is that this would tremendously ease whoever wants to create a fake as it'd be possible to engineer a fetus and his growth environment to reach the required characteristic. This may have severe ethical problems, but if we are talking about states under war that think they can use the fake information to win the war, then it wouldn't be such a far fetched possibility and needs to be considered and you can't make sure that your enemy state didn't use it.
Any not-currently-known technique or scientific knowledge
Lastly even if we find something different from DNA and C14 that we can't currently do (but we have reasonable certainty that it can be used to identify the body of a time traveler), it would still be not viable, as pointed out by Максим Корчагин under an other answer:
Scientific discoveries definitely fall under #2 and #8(because the time between actual findings and theory being published might be quite big).
And in this case we would be using a scientific or technological discoveries to satisfy #8, which wouldn't comply with OP.
To conclude neither information, objects (can be stolen) or characteristics of the body can be used to guarantee that a certain person is from the future as required by OP, unless we ignore stealing information from an actual time-traveler.