How can you detect a Doppelganger?

First, within D&D 5e, what magic, items, or special abilities would allow player characters to detect a doppelgänger?

Second, and perhaps not easily answerable, what types of clues or cue might give away a doppelgänger without the use of magic? Or how as a DM might I give players a chance to notice one?

There is a doppelgänger within the party and I’m struggling to figure out how the players might figure it out. And once they do, how they might track the doppelgänger assuming it escapes.

Thank you.