Will the black light only show urine on the fibers, or will it also be apparent on the backing and pad?
A black light illuminates phosphors, especially uric salts. There are other products containing zinc or fluorescent dyes that will also fluoresce, but the distinct pattern animals leave are a giveaway regarding pet urine.
Pet urine is fluoresced as bright yellow spots, streaks and drips. It will show damage on the backing and pad. A black light will not detect surface urine after the carpet has been thoroughly cleaned.