I'm starting T on Monday, getting my name and gender changed on my ID by January. I'm starting college as a new person. In the future, is there a way for anyone to ever find out I used to be a female? I am hoping to change my birth certificate in the future after top surgery. I do not want to be an out transsexual, I want to simply be a man, and live my life as a man. I'm 18 by the way, so it's a perfect start.
Also, I am training to become a competitive bodybuilder. Will my old identity ever be found out? Through documents or something?

Depends how recognized you become as a bodybuilder. If you're the next Arnold Schwarzenegger, it will come out somehow. But you could probably use that to your advantage. Don't think there are many well-known ftm body builders around.

If your medical history and name change come into light then you could get outed.  Even applying for a job they are likely to look through your past records and might notice something.  But in terms of the average person on the street, nope.

if you meet someone you knew from before your transtion, or someone who knew someone-of-someone,blabla,
yeah then it could happent.

But even if someone find out, so what, it not a shame being trans, neither is really peoples busniss.

In terms of being outed via a background check, etc. since you are transitioning at 18 it is less likely to be an issue than someone who transitions just a few years older. If you've never had credit under your old name that will probably save your reports from outing you (in fact if that's the case and your old name appears than that probably means something more serious, as in there was likely some identity theft when you were a minor!). If you've never worked under your old name, your school records have been updated, and you haven't done anything criminal before your name change then you're probably off the hook as far as employers are concerned (I discuss that issue more in detail here).

If you're going to be a bodybuilder you may have other issues, like if a physical has to be done or you become famous and word leaks (I don't know much in that area though).

As young as you are you can avoid a lot of history checks if you want to. Your doctors and health insurance are the ones most likely to keep a record of your previous gender designation. If you have no health problems you might could even break that trail if it's worth it to you to do so.

If you want your credit reports to not out you it would be best if you got your social security gender marker changed as soon as possible. But in everyday life there's no reason to think anybody will even wonder about you.

"you might could"

Thanks, Felix! I haven't heard that since I lived in West by God Virginia! Hugs, Devlyn