Having kept sheep I would say it is almost impossible to be certain you have no fly strike until it is almost too late.
For those unsure typical signs are restlessness - running a few steps and stopping often with head down, nibbling at themselves, scratching on posts or branches and lots of others too insignificant to mention.
As an old shepherd caught leaning on the gate by his employer said "I be a'watching of your sheep Master" Look at them just grazing. Any sheep which is doing something unusual especially if they are away from the flock has usually got something wrong with it
I am very "anti" sprays and chemicals, but it is about the only thing I have found which gives real peace of mind. Autumn is a real problem. The days are damp and warm and you don't need many flies to start the damage.
I think flystrike is the worst thing about keeping sheep
all the best and don't let it get you down to much - It happens to the best of shepherds
Sue
Dark Brown Eggs