In successful case, you'll interact with them for a new job and never hear back again (they got you a dream job). If you're hearing from them again, what was wrong with the first job they got you?
You can't go by recommendations. A recruiter may have been decent when working with a certain client and then be shitty with another client.
One recruiter asked if I wanted a job. I said no. He asked if I knew anyone else who did. I gave him a name. He said, "Oh, he works for X? I recruit for them; I can't present a person from there for somewhere else." I was really impressed. A recruiter with ethics? I made a note - next time I need a job, call this guy.
Sometimes you can tell by the way they talk to you. When you say, for example, "I mostly do embedded software", do they know what that is? Do they know that they should stop sending you web front end jobs?
As I said, not 100%. But if I can deal with the best 20% of recruiters (with an occasional mistake thrown in), that's still good enough.
If they ghost you after promising X, Y, Z, then I don't interact with them again. They aren't reliable.
"In successful case, you'll interact with them for a new job and never hear back again (they got you a dream job)."
Clearly if they landed you your dream job they didn't ghost you in the middle of the process, right? They must have followed through? Communicated professionally?
Likewise, while I've gotten some pitches that aren't in my domain, and 2 that weren't even software engineering gigs, I'd say 90% of emails seem to at least be loosely in the ballpark, and of those half of them seem like "okay, it at least got the right kinds of positions). Most of the rest tends to be web development for some reason, though (again, I don't have any specific request on my LinkedIn).