I would add to what Amy has stated that you could also use the sign "TRUE/REAL". It would depend on the context but if you were responding, when you say "I think so" you are saying something is "true" or "correct". Signing TRUE means in this context. "I think that is correct" or "I think that is right." You can also use the sign FEEL++ with nodding your head. This means "I think so". Often the sign "FEEL" is used to express mental activities. Like "What do you think?" You would make the sign FEEL++ with a wh-question expression. It would mean, "What do you think?"
So there are a few different ways to sign "I think so" depending on what the context is and whether it is a positive, negative, or questioning situation.
Hope this helps.
Bill