Pick up the pot and look under it.
Is there a catch tray? Is it attached to the pot? If so, tear it off. Then check it for muck and wetness. Is there moisture hanging out under the pot?
NExt, look around the room for water. If you don't find any, look outside. My friend has a bunch of non-pot houseplants in dirt, and he is near a paper mill (muck and wet, oh my!), and he's got fungus gnats coming out of his ears.
Finally, hang yellow sticky traps to catch the little shits.