Say you are 30 years old, and something happens and you wake up and find that you've been transported backwards in time to when you were a teenager [you were inadvertently sent back, you do not have control over the process or if it happens to you]. Physically, the whole world appears to be exactly as it was the first time you were a teenager: your surroundings are identical and your outward appearance is identical to how it was. However, your brain is different in the sense that you have your 30 year old pre-transportation brain: you remember everything from your life that happened up to when you were 30 and were transported back in time.
From talking with people around you, it seems like they did not have a similar experience, i.e. they were not transported backwards, so they do not realize that a jumpback event occurred. However, you cannot assume this is true of everyone, i.e. it is possible that some other people were also jumped back. In fact, you should assume that at least some other people also were sent back, you just don't know how many or who they are.
What are some very concrete steps you need to take to conceal the fact that you were jumped back? This involves 2 parts: hiding the fact from other unsuspecting people, and hiding the fact from other people who also jumped back.
How could you identify other people who jumped back without revealing that you were jumped back?
**It's true that if you had technology to see the structures of a person's brain, you could determine that a given brain is unlikely to belong to a person of a given age, and use this to classify people as jumpers/non-jumpers. But I'm mostly interested in other methods besides physical brain structure. E.g. taking advantage of assumptions that a jumper might have based on world events they experienced, such as "accidentally" knowing who Obama is even though the year is only 2002, or "accidentally" knowing that some sports team won X championships when in the present year they've only won X-2 championships, or Googling movie or song or book names before they were published, etc.
***Also, how does this change if we don't assume that the jump increment is the same for all jumpers? E.g. you may have been jumped back by 15 years, but another person may have been sent back 5 years, or 50 years.