The first sign is the cocoons, either inhabited or not; half-inch long slivers of white silk. You want to check for moth out in the sunlight, as the adults can't abide sun. As you disassemble the tumbled pile of locks, adult moths scamper or flit away.
The dried frass becomes a very light floaty dust of chewed wool bits. It gets into your clothes and into the creases of your skin and itches like a fiend.
After you get as much dust, as many cocoons, and other detritus out of the locks you can either freeze or boil them to salvage them. I'd frankly rather find that the whole batch has been ruined, as handling the dusty chewed-on locks is very discouraging, but naturally dyed wool for my airsaid project is much too valuable to throw out.