You are gay if you are sexually and/or romantically attracted to the same sex. That is the only way you can know.
No there is more to being gay then the undergarment you choose to wear. If you like them more power to you.
When my wife and I went to a secluded Island in the Bahamas I wore thong suits. They’re actually very comfortable, believe it or not! If you are a 15 yr. old boy, wearing thongs doesn’t “make” you gay. If however you are attracted to the male body and not the female body, that might be an indication. HOWEVER; at such a young age it’s not definitive. As you grow older and find that girls don’t turn you on, and you masturbate to pictures of men, and not women, then you might be gay or Bi.