Summarize the following:
Examine all the drains in your home and any area inside or outside the home that has standing water. Any area with multiple drain flies is likely a problem. Luckily, drain flies do not stray far from the place they have chosen to make their home, so it is rare for a drain fly infestation to spread to all areas of the house. This is especially true if you catch the problem early. This is the first thing you must do to verify that the problem is coming from the drain itself. You might actually be infested by another type of fly drawn in by rotting fruit or other food sources. As a result, you should verify that you actually have drain flies in your drain before taking measures to remove them. Stick the tape over the center of each drain with the sticky side facing down. Do not completely cover the drain with tape. A covered drain will prevent flies from flying up, leaving you with no evidence. If you spot flies on the tape after removing it from the drain, drain flies are the most likely culprit. Even if you do not find any drain flies on the tape after the first night, you should repeat this process for at least four nights to account for discrepancies in the breeding cycle.

Summary:
Identify the problem areas. Dry all the drain areas before going to bed. Place a strip of clear tape over each drain. Check the tape in the morning or after a few days.