It's not always easy to tell when an animal is in distress, pain or illness. But the following signs are indications your cat needs a veterinary check up.

stops interacting with you and may avoid eye contact

stops grooming. Matted coat.

a hunched posture.

dribbling or wet chin.

incessant licking, over-grooming, or self mutilation. Bare patches or nibbled areas of fur.

vocalizing, which can include crying or groaning

hisses when approached or touched

attacks or bites when approached or touched

sits very still, lies rigid or is non-responsive

stops eating. May approach the food, eat a tiny amount then back away.

stops drinking. Drinks excessively.

toileting in the wrong area, possibly because of difficulty getting into litter tray or reaching litter tray by going up or down stairs.

changed deposits in the litter tray. Nothing in the litter tray. Lots of small deposits. Blood in the urine.

reluctance to stretch or jump or a general reluctance to move with increased sleeping

lameness

weight loss or weight gain

lumps or bumps on the body. Sores or wounds that don't seem to heal. Crusting skin. Unexplained bleeding.

breathing difficulties.

changed habits in general. "Not himself somehow."