HOW CAN I TELL WHICH OF MY FISH ARE NIPPING AT MY NEW ANGELFISH ?

WHEN IGOT HIM HE HAD BEAUTIFUL LONG FINS. NOW IT LOOKS LIKE HE'S HAD MOST OF HIS TAIL FINS NIPPED OFF. HOW CAN I TELL WHICH OTHER FISH ARE DOING THE NIPPING IF I DON'T ACTUALLY SEE THEM? I HAVE 3 SMALL NEON TETRAS, 1 SUNSET PLATY, 1 MICKEY MOUSE PLATY, 1 GUPPIE, 1 LONG FINNED TETRA, AND 1 CORY CAT.