How do veterinarians/doctors diagnose a Charley Horse in someone /pet/ livestock who can't communicate?

They don’t, unless they happen to be observing the “someone /pet/ livestock” in question when it happens to have a spasm, and they recognize it.

I would not expect a “Charley Horse” per se to be really a medical condition to diagnose. My impression is that, at most, it might be a temporary event, or a symptom of some behavior caused by some other situation or behavior.

A “charley horse” is a simply a muscle spasm. And anyone with a discerning eye can spot it. If I, as an untrained observer can run my hand over a muscle spasm on one side of the body, then do the same on the other side and find the spasm, so could anyone else.

I know this is true because I did this as a child. A physical therapist working on my mother showed me how to do it.

Specifically, the muscles of a spasm bunch up a bit and get hard. (edited)

Is that the same as a cramp like so many of us get in our arches and calves? I would know someone had one if I saw them doing what I do when I have one which is jump up and walk walk walk until it’s gone. A couple of curse words also helps.

I’m no expert on muscle spasms, but I have noticed that if i move my leg one way the spasm gets worse and if I move it the other it doesn’t, or slowly goes down.

I gave my little brother so many charley horses that it left him unable to speak.
Then I moved on to giving him wedgies lifting him to the ceiling.
No Doctors were needed.