When they've been recently clipped it's easier to be sure. Watch the sheep for a few minutes; when their digestion is working properly you can see (or if you have them held you can feel) the 'rumen wave' progress across and along their left flank. It should do this every 30 seconds or so, and should be accompanied by a belch as they expel the rumen gases.
If there's no rumen wave, and their left side is distended and hard, then it's probably bloat.
The yoghurt / bicarb / ginger /oil mixture really does work, and massaging the left side will also help.