Other signs to watch for are flies buzzing around a particular sheep - even if you can't see anything, chances are that sheep's been strucken.
And once the horrible things hatch and start munching, there's a wetness and a smell. The wet can sometimes be seen, sometimes just a slight discolouration of the fleece, and/or something about how the fibres lie.
Someone up-thread talked about sudden darting movements of the head, or foot - always watch a sheep that does this, if they do it again they have probably got something going on somewhere.
I've no experience of internal strikes, and don't know what, if any, external signs of this there might be, apart from a sheep that's clearly a bit miz - ears drooping, lying down reticent to move, hanging head and not grazing.