The immortals apparently have a radar sense that tells them when other immortals are nearby. That only tells them a fellow immortal is near, not which person in a crowd is immortal.
How do any immortals recognize other immortals?
I only saw the original film so if you know the answer from the subsequent films (or TV series), please share.
The very short answer is that they don't, at least not in the original film/s. They know that another immortal is nearby (courtesy of their 'immortal radar'), but until one or the other reveals themselves as being ready to fight, they're given no specific indications of which individual is an immortal although the original film script suggests that there may be some indication of range.
Striding past rows of cars. FOOTSTEPS ECHOING in the Garden's underground garage. MacLeod senses a presence. It's very close
MacLeod, for example, fights a Zulu warrior in the Highlander novelisation. Kastagir tells him that the man is an immortal and MacLeod believes him. When he turns out not to have been an immortal, Kastagir has a good laugh at MacLeod's expense. It's clear that Kastagir's continued presence means that his 'radar' is effectively non-functional.
MacLeod was stunned. Hadn’t Kastagir said that this man was an immortal? Kastagir! MacLeod looked around for him, but it wasn’t until Cetewayo wanted to talk to MacLeod that he saw the Ethiopian. He was standing by the king.
Kastagir cried, ‘Cetewayo thinks you are a fine warrior. He praises you.’ The king nodded, as if to confirm.
‘Thanks a lot,’ replied MacLeod, sarcastically. ‘I could have killed him that way half an hour ago, if you hadn’t lied to me. You said the warrior was an immortal.’
‘A joke,’ smiled Kastagir, and though he obviously did not understand the words, Cetewayo nodded violently and shouted something in Zulu. The nation responded warmly. Kastagir said, ‘You know what great humorists us Zulus are.’
Obviously immortals might also recognise each other in other ways;
Both men had been wounded many times during the melee, but still sat upright in their saddles, one at either end of the valley. Between them lay hundreds of bodies: the dead on either side.
The dark one, Kurgan, called to the Mongol ‘There can be only one!’ but whether the knight from the far Orient heard and understood was not evident from his posture or any gesture.
In this franchise all immortals have what's known as The Buzz, a type of extra sensory perception that "goes off" in the presence of another immortal. It seems to be a sound or vibration that happens in their head, similar to Peter Parker's Spider sense. However, unlike that early warning system, its very, very general; it seems to work within about 50 -100 feet, but isn't very specific; two unknown immortals may get a "Feeling" about someone, having grown accustomed to their fellows showing up randomly, and knowing what signs to look for... but they wouldn't [by all indications] be able to "pick each other out" specifically of of a crowd, sight-unseen.
The Buzz also doesn't seem to produce any other physical effects, nor does it get louder, stronger or weaker as an immortal gets closer or further away. Effectively, it's just an early warning system that let's them know another is near, so they can't be sneak attacked. As a narrative tool, it's pretty clever; its a nice way to even the playing field and wonder why the Game didn't end centuries before. I think, though this is speculation, that this is an more refined extension of the "Connection to everything" that Rameriez was trying to teach Connor MacLeod to explore when he first became an immortal.
William Panzer was part of Davis-Panzer Inc, the production company behind the Highlander franchise. He co-wrote screenplays for three Highlander films, and was the Executive Producer for Highlander: The Series. In an interview on the 2001 bonus material of the series season 1 DVD, Panzer defines the Buzz as:
a concept of Immortals being able to sense each other's presence from a reasonable distance. We called it the Buzz. That word was never used, but that's how it was featured in the scripts.
It's further expanded upon in the Fan wiki, based off interviews and DVD extras: https://highlander.fandom.com/wiki/The_Buzz
Also in the episode "The cross of st. Antoine" Amanda hides in a church to trick Armand Thorne/John Durgan into believing MacLeod is there.
Even though Macleod and Thorne met before Thorne doesn't recognize that it's not Macleod.
So from the Buzz alone they cannot recognize immortals. They'll need a visual clue, either by recognition of the other immortal from an earlier encounter or through behavioral clues like meeting eachothers gaze.
