How do synths and humans differentiate in Extinction?

Their fashion and clothing!

If you look at how synths are dressed before/during the War of Decommissioning, they're all wearing very basic clothing (i.e., simple fabrics, no design/patterns) and their hairstyles are something from the 40's and 50's.

Synths just before the War of Decommissioning.

If you then look at how humans were dressed, they wore much more modern clothing and hairstyles, something akin to how people are today. Although humans aren't really shown throughout the film, we do later find out that the synths took the homes of the humans after the war, and, as a result, started wearing their clothing and taking on their hobbies (e.g., Alice starting to paint).

Synths wearing human clothing after the war.

Alice when first entering her new home after the war.

Alice painting, waiting for Peter to come home from work.

Another element that strongly evidences this is Peter's pocket knife. At the beginning of the film we see him pick it up when first dressing for the day, and then later when he realizes that he's really a synth, we see a flashback of Peter, Alice, and the two girls first entering their home and Peter picking up the pocket knife.

When Peter leaves Miles (the young human boy) near the end of the film, he hands Miles the knife and says:

PETER: You know what, here. It was never mine to begin with, but thank you.

Peter handing Miles the pocket knife, along with Peter wearing the watch that wasn't his to begin with.

This is never outright explained within the film, and really it's unimportant to the story.

The synthetics appear to know and recognize each other as synths prior to their memory wipes. Because of this inversely, any 'human' who's trying to kill you is probably a synth.

The other explanation is likely that humans are simply aware of who the synths are. They are built machines after all. Everyone is likely aware who is a synth or not simply because they are products placed in specific roles within society.

Of course that may only explain away the very beginning of the W.o.D. Before confusion and disaster eventually set in... it's likely as the war continued, humans created tools to visibly tell the difference.

But it's never depicted of synths pretending to be humans either. There were no terminators attempting to infiltrate the humans that we know of and as such, we never learn how hard it is to tell the difference between them.

We only learn from the solider that the generations later from Mars learn everything about them for their upcoming 'invasion', and also know the difference.

However for us, as such it appears you can just.... tell.