I think the most important part of recognising illness is to know your bunny’s normal behaviour. For example one day when I got home from work, all I did was peek in on the bunnies and new straight away that Apollo wasn’t feeling well. I knew because he was laying somewhere that he never normally lays.
There can be many indicators, laying down more than normal. Laying in different positions or places to normal. Laying down but constant shifting like they’re not able to get comfortable. My previous bun, Bandit, had a stomach pain pose, many buns do it but I find Apollo lays in that position even when he is feeling well. The front paws are pushed forward, the back paws are tucked right up under the bum which pushes the bum up, arches the back down and presses their belly to the ground. Loud tooth grinding/crunching is a sign of pain.
If you ever suspect your bunny isn’t feeling well, the best thing to do is the treat test. If they refuse their favourite food, you know they aren’t feeling well. I’ve found different levels of refusal depending on how unwell they are feeling. If they feel really unwell, they try to move away from my hand offering the treat. If they’re feeling mildly unwell they’ll just sit there but won’t show any interest in the treat. If they’re unwell but not too badly they’ll show interest in the smell of the treat and will seem like they want to eat it, but they won’t eat it. A bunny that doesn’t want to eat or that isn’t pooping or peeing, is a sick bunny.
Most of the awkward laying positions can be attributed to GI stasis, other things too I’m sure, but GI stasis is probably the most common thing you will come across. A tilting head, lack or balance or trouble walking can be a sign of an inner ear infection or E. cuniculi. Hind end paralysis can also be a symptom of E.c. but also the start of a condition known as floppy bunny syndrome.
There are many other things I’m sure, but that’s just a few things to look out for.