Used the bucket and food to check all of the others.
I find that it's easier to spot problems when you don't have the bucket of food. There is a lot to be said for just standing and watching for 10-15mins. Once your sheep get used to you just "being there", they behave as they would do when you aren't there and it's much easier to spot one that is behaving oddly (and in my experience every sick sheep exhibits some sort of behaviour that is not quite the norm, even if it's subtle). When there is food involved they may well quite forget the fact that they are in pain or being irritated by something and gallop happily over to scoff the food.
I have BWM and I don't think I could spot a strike in the black wool, I find it hard enough to spot a dirty bottom (not that I get many of those).
One thing I would advocate if you have sheep with horns - stockholm tar. I bung a squirt on the heads of all my rams/ram lambs whenever they are gathered. It can last a good couple of weeks and it's dirt cheap. I've even splodged a bit on their rumps too when the flies have been really bad, when I ran out of crovect - okay, so my sheep are black so it doesn't show up, if you had white sheep you might not want to paint them black
. I would even have been tempted to spodge it on myself yesterday as I got about 30 horse fly bites whilst hay making!