system
Given the following post, respond as if you're commenting on Reddit:
human
[Fallout 4] How did the people of the Commonwealth get so good at discovering synths?

 I just came across a group of 3 people in the north eastern part of the Commonwealth. They were threatening to kill what seemed to be a synth who had infiltrated them for weeks. I, being the benevolent savior of Boston that I am talked them out of such a heinous act. 

 Buuut curiosity got the better of me and one quick save later it turns out they were right! Jules had a synth component on him! It got me thinking. Most of the time when an accusation of a synth took place they were, more often than not, right with the Mayor and the Caravan leader's daughter being prime examples.

 So how can the people of the Commonwealth tell that their loved one has been replaced by a synth considering the difference the technology invested in the Institute? 

The closest thing would be the Covenant who have a test of sorts but they've even said it's still under development so they can't have given out to the public. Also, honestly the test itself is just the S.P.E.C.I.A.L test from fallout 3 which puts an even bigger shadow on it's validity.
gpt
We really don't know if they're that good at it.

Sure, that one group was right, but how many were there that were wrong? For every real synth there are who knows how many innocents killed for being suspected of being a synth. 

Not to mention people are already super paranoid, so any sort of unusual behavior will set people off and make them think "synth!"

And the Covenant test is garbage. For every synth they find there are three normal people who get a false positive. And we have no idea how many synths managed to pass the test. So it's actually worse than just flipping a coin.