Is there a way to tell if a girl is wearing a thong?

not really, that's the point of a thong...

The only thing I can think of is if you can see pantyline then obviously she's not.

Depends on the pants. If jeans or any thicker material, then no. Unless she pulls them high up and when she sits down you can see them. If yoga pants, then yes, because normal panties will show through and you can see pantie lines. But then again, underwear now can be made with so many types of materials, so not really unless you see them.

look at her panty line ?
o.o
for the most part...not really...

Unless you can see it, then no.