How do i know if im adopted?

Question:

How do i know if im adopted?

I am 22 years old and my youngest brother is like 38. I just find it odd. I have always been the black sheep. I am wondering how do I find out if I'm adopted? I mean they are still gonna be my parents regardless. I am just curious. I was online looking up my family tree and my brothers came up an ansestery.com but there was no record of me ever being born. I have seen my birth certificate but I recently found out that your original birth certificate can be hidden to where the adopted child will see a birth certificate with the adopted parents as the birth parents. I am just wondering about this. I live in NC. Thanks for any answers.
p.s. My parents deny it. So asking them is not an option because I already have. I asked this question earlier and someone suggested I was my brothers kid ans I've always noticed my youngest brother was always nicest to me and spend alot on me at christmas. My mom always tells me i look like him. But i Don't know. I cant afford to get a dna test.

Best Answer - Chosen by Asker: dear God in heaven...
Ancestry.com DOES NOT INCLUDE EVERY PERSON IN THE WORLD... nor does the fact that you don't find YOUR birth in there, prove in any way that you were adopted!!!
Birth records of living persons should NOT be online! How or why your brothers are there is unknown... it might be as simple as the birth records for one county were extracted, and the neighboring county (where you were born) were not.
It is completely standard for birth certificates for adopted children to be amended. Thus if birth records were "complete" for wherever ancestry got them... yours would show up also, as the amended record. The original in an adoption is sealed, and stays that way, dependent on state law.
Your parents have denied it. Unless someone else disputes them... your option right now is to accept that they are being honest. And the one brother may simply be a "nicer" brother than the other.