I'm sure Baz will come on with more but some of the signs are twitching, stamping, scratching, looking irritated and/or introspective, uncomfortable and unhappy. Maybe getting up and lying down repeatedly. The tail may wiggle like it does when they poo but repeatedly and they are not pooing. Most likely sites are around horns (if they have any horns or horn buds), around backend, in the feet, any where they are dirty and/or have any kind of wound. Underneath would be an unusual site in my experience. Once the maggots are established you get grey wettish streaking of the wool, but if you are up close and personal with your two every day it will not get this far advanced. In fact, you would be pretty likely to see the flies taking an interest in a particular spot on one of them and can clip it out and clean it up before anything even happens.