How could i tell if the iris was damaged after a thermal burn?

See an eye doctor: You need to see and eye doctor to determine this and to check for any other kinds of eye damage.

Unlikely: The iris is inside the eye. For a thermal burn to damage the iris, it would have to burn through the outside of the eye, causing severe damage (and pain) with a need for emergency treatment. You probably just had a superficial burn of the cornea or conjunctiva which does not harm your iris.