It's not always easy to tell when a cat is in distress, pain or illness. They do not show usually show pain by crying etc. They protect themselves against predators by not making any noise or showing any bodily change like limping. But the following signs are indications your cat needs a veterinary check up in case it is suffering from pain or illness.

Stops interacting with you and may avoid eye contact

Stops grooming. Matted coat.

A hunched posture. Tail and paws tucked away.

Dribbling or wet chin.

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

Vocalizing, which can include crying, yelping 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 or 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. Spends a lot of time going to the tray.

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. Coughing. Wheezing.

Changed habits in general. "Not himself somehow."