If you spend a lot of time watching your sheep then I would expect that you would tell the difference pretty easily. They will not be relaxed and cudding, but quiet and still, inward-looking, in discomfort. Not an easy spot if you just glance at them once a day, maybe, but should get alarm bells ringing in anyone who spends a fair bit of time just watching them. (One of the reasons you see old sheep farmers leaning on gates. Still working

)
The other thing to take into account is the likelihood of bloat. Once bottle lambs are weaned, the risk factors are really about too much / rich food too quickly, and or any food which might ferment. If they're on the same field they've been on for a while (and the grass hasn't suddenly gone mad after some rain and sun), it's fairly unlikely.
When you move them onto lush, fresh grass, especially if it has lots of red clover, then keep a closer eye for the first few days.
If they break into the feed shed and gorge on cake, keep a close eye.
If they escape into the orchard or veg plot...
And so on.
Relax, enjoy your sheep; enjoy watching them and you will be fairly sure to spot anything amiss.