By asking them out. I'm sure their response would let you know
with an honest smile on your face
Just ask the person out.
If they refuse, then does it really matter why?
Regardless of their preferences, you'll know to move along.
I would go with ask... and if your nervous to ask straight out, ask if they have a girlfriend... or start joking around about passed relationships back and fourth to get a feel... OR point people out and ask if he/she thinks there hot... its really pretty simple.
Just how does a 13-15 year old ask someone out to begin with? 2nd, what difference does that make if you're asking them out or not...you obviously like them enough to ask...you're under age for determining their sexual orientation...anyway...
the odds are for you...if they are straight or bi you can still ask them.
As them if they are Gay, Bi or Strait. Then proceed. Not rocket science.