How do you detect a doppelganger?

For plot purposes I replaced an NPC with a doppelganger (one of the advanced “rumored” kind that can steal memories and skills). As this quest progressed, I found that it was actually rather difficult to detect a doppelganger. In this case, he was a cleric in an order of paladins & clerics, so detect magic was out of the question. Supernatural abilities can’t be dispelled. And since the doppelganger had memories and skills (and, for plot purposes, had forgotten he was a doppelganger), a goof up or mistake in “performance” was unlikely. The party got through it and figured it out, but it did have us wondering:

Is there a spell or ability or some obvious racial “tell” that allows players to detect a doppelganger?