You will be able to film a gait so you can replay it, recognise the head movement that goes with lameness, and start with the two free checks that explain a good share of sudden limps.
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An obvious limp gets noticed at once. A gait that shifts over six months almost never does, because you are watching it every day.
Flat ground with grip, a steady pace, ten seconds from the side and ten seconds from behind as it walks away. Ask someone to hold the lead so you can film without walking yourself.
Playing the video back slowly shows what live watching does not. It is also the only way to compare in three weeks, when you will no longer know whether it looked the same.
On a front leg, a dog spares itself by lifting its head as the painful foot lands and dropping it onto the sound one. The nod is obvious in slow motion and invisible in the moment.
On a back leg, watch the hips from behind: the sore side lifts away sooner and the stride is shorter. Do not try to name the joint, just note the side.
Trouble that is worst on getting up after a long sleep and fades after a few minutes of walking follows one pattern. Trouble that appears after the walk follows another.
Note the time and the setting for a week: on rising, after exercise, in cold weather, after the stairs. That diary is worth more than your description of the limp itself.
Cats hide mechanical pain for a long time. They do not hobble: they jump lower, break one leap into two stages, give up the top of the wardrobe, sleep on the floor where they used to sleep high up.
Litter tray accidents belong in the same box. A high-sided tray becomes an obstacle before it becomes a refusal, and a low-sided one sometimes settles in a day what looked like bad habits.
These small surrenders can be dated. Write down when the last time happened, because that is exactly the detail nobody recovers afterwards.
Limping on a leg and refusing to put it down are not two degrees of one problem. A limb held up all the time, or an animal that will not stand, is a same-day call.
Do not massage it, do not move it about, and give no human painkiller: several of them are dangerous for dogs and cats, even in small amounts.
When a limp appears out of nowhere, look between the pads and look at the claws. A thorn, a piece of grit, a shard of glass, a claw broken back to the base account for a fair share of sudden limps, and a phone torch finds them.
This does not replace an opinion, and it does not apply to an animal that screams when you reach for the foot. But it takes thirty seconds, it costs nothing, and now and then it saves a whole appointment.