So, keep in mind that I believe it makes no difference whether a person was born a Human or manufactured a Synth. I don't think there's moral rectitude incurred by being Human, or moral turpitude by being a Synth. Therefore, I have no inherent interest in a Synth detector.
But... there is this thing I've noticed.
When you do the "Vault-Tec Workshop" quests in and around Vault 88, you gain access to refilling crates which spawn Pip-Boys. I discovered what I'm about to relate because I started giving Pip-Boys to my couriers so they could reliably get to their destinations and report any issues they had along the way. Yes; that's in my imagination, what you might call "role play." The point is, I was giving Pip-Boys to a bunch of NPCs.
And most of them would immediately equip the device, causing it to vanish out of their inventory. But some of them? Some of them wouldn't. It would just sit there in inventory. Why, you ask? Because there's a hidden inventory / gear slot which, when something is equipped in it, causes that item not to appear in the inventory list. And the thing that's normally in that slot, which causes the provisioner not to equip the Pip-Boy?
The synth component.
So if you give an NPC a Pip-Boy, and they don't equip it? They're a Synth.