Yes its true! Go and look at any really healthy sheep & the hair on the face is smooth & shiny. If they're at all under the weather it looses its shine and just looks a bit flat, just like hair in people. The fleece can show changes too but these are much slower & reflect the animal being below pare for a while - often the group are affeted eg liver fluke, trace element problems. Believe me I'm often called out to flocks where the only sign is the faces have lost the shine. Invariably the shepherd is right & there's an underlying problem grumbling away.