Yeah, this can be a tough one. First I would check a reference such as the LEED Reference Guide for BD&C for typical SRI of whatever type of roof it is to see if it is in the running for this credit.
Second, locating cut sheets for the product installed is the easy path here.
Your other options (more difficult) include removing a piece of the roof and sending it to a lab, or testing in place. These options were explored in a comment thread here.
I am not sure if there are any other options beyond that—which as you point out makes it tough for an existing roof. Anyone else have thoughts?