There's a Clause for that
Well, not to see the true form behind the Skinmask, but Spellbound Autumn * lets you detect supernatural beings when you use it. You may also be able to pick them up by scent (such as with Fang and Talon ** or the Cleareyes kith blessing), via Kenning (this is a poor solution) or by touching/dealing minor damage with cold iron, which is portrayed in the various bits of fluff to have a rather visceral reaction. If you have a pledge or another relationship with the subject you can use Contracts of Potential to identify them through those pledges or relationships, and if you know that they've broken specific oaths you can twigger them out with Oath and Punishment.
And then the psychological angle
Changelings are fundamentally broken people. Mirror won't solve the thousand-mile stares or the aversions to certain subjects. A heartless or desperate Lost could attempt to provoke a reaction that will twigger the disguised Lost as not being human, or as having had similar traumas.