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.