Most should say right on the label - all mine give a continuous and a burst rating, some both in C and amps

Lipo performance has improved over time. Cells are getting better and that means more amps output which increases C rating.

My integy 1500mah 2S are 15C rated continuous and 20C burst. I have a 2S Flightpower EvoLite V2 1600mah and its 20C rated continuous, 30C burst. Its a better, newer batt than the Intergy. It cost more than the Integy too.

Your 1500 is either older when that was the best a cell could do or was made that way to suite some specific end use that just needed that amount of current.