Other contributors have said "How can you tell if a girls have a thong on?" is the same question as "How do you tell if a girl Where is a thong?" If you believe that these are not asking the same thing and should be answered differently, click here
theres the obvious, if she bends over and you see it. if that doesn't work, look to see if there are lines through her pants lol also when she walks you can sometimes see her butt jiggle:)