On a serious sheep note (I can't be bothered wasting my life commenting on other peoples comments, no matter how much I want to...
) .... you do need to find a better way of inspecting your flock. Also, if you think something is wrong then it usually is, so act immediately, don't put it off, no matter how inconvenient it is for you (we've all been there, and wish we'd acted sooner).
What I would suggest is that you teach your sheep to come when you call. It's a bit like teaching them to come to the shake of a bucket of nuts but works over a longer distance - also you can ride on the quad to get a bit closer to them if you need to, to call them in. Then have a small handling area into which you can easily funnel them. If I recall there is a thread on here about teaching sheep to come when called - its easy to do, especially if they already know you have nuts for them. Now would be a good time to start, especially if you've recently lost your "lead sheep".