Kousa, it can actually be difficult to tell. Fungus gnats can also carry the damping off fungi and spread them. You may see brown, water soaked lesions on the seedlings with damping off, or signs of actual fungus. Some plant seedlings just keel over at the base. Some may just wilt.
Amanda, putting gravel in the bottom of containers actually makes the drainage worse because water doesn't move down into coarser material until the finer media on top has become saturated and can no longer hold any more. It sounds counter-intuitive but it creates what's called a "perched water table". I actually tested this myself once and found it to be true.
