First Sign: Small, dingy grey flies, flying around plants aimlessly. If these "flies" are also seen coming out of the soil around your plants, you probably have Fungus Gnats.
Special Species Notes: Fungus Gnats can be told apart from Whiteflies, a much more serious pest, because they're dingy grey instead of pure white. Fungus Gnat larvae, small worms found in the top inch or two of your soil, feed mostly on organic debris, fungi, algae, etc. While doing this, they often nibble the roots of young seedlings. Your plants will often outgrow them, so they may be more of a nuisance than a real threat to plant health.