A bathroom exhaust fan does one job. It moves humid air out of the room and through a duct to the outside. When it loses pull, the room keeps its moisture longer, and the effect shows up on the paint, the grout, and the framing above the ceiling. The early signs are small. Write them down before they add up.
Start with the time it takes to clear
Run a hot shower for about ten minutes, then stop it and time how long the mirror stays fogged with the fan running and the door closed. Keep the conditions the same each time: the same shower length, the same water temperature, the same door position. Note the result in your log next to the install year from the label inside the housing. If the clearing time grows over several checks, the fan is moving less air. That is the first thing to record.
Listen to the housing
Run the fan with the grille in place and note the sound. A steady hum that has not changed is normal for the motor. A new rattle, a buzz that comes and goes, or a wobble that follows the fan speed points to something loose, a worn bearing, or debris on the blower. Also note whether the sound changes on a cold start. Fans often sound different when the room is cold, and that is worth logging too.
Check the grille, the cover, and the room
Look at the grille from below. Dust and lint that collect on the cover reduce airflow and make the fan work harder. Clean it with a vacuum brush attachment. Check whether the cover sits flat and whether its spring clips still hold. Then look at the ceiling and the walls around the fan housing. Faint staining, paint that bubbles near the grille, or a musty smell after a shower are signs that moisture is getting past the fan rather than leaving through it.
Check the outside end
Go outside and look at the exterior hood. Watch the flap while the fan runs. It should lift freely and close fully when the fan stops. A flap that sticks shut, stays open in calm weather, or is blocked by leaves, nests, or lint means the path is restricted. Clear what you can reach safely from the ground. Do not climb to reach a high vent.
What to leave to a licensed hand
Duct runs in the attic, the wiring at the switch and the fan, and any replacement of the motor or the unit belong to a licensed electrician or a contractor. Ask a professional to look at a duct that sags, has gaps at its joints, or ends in the attic rather than outside. Ask about the wiring if the fan hums and the light flickers, or if the switch feels warm. Ask whether a replacement can match the airflow you need, and whether the existing duct size will carry it.
Plan from the install year
Read the install year from the label inside the housing. If the label is missing, use the year on any permit record for the bathroom or the year the room was last finished, and mark it as an estimate in your log. Treat that year as a planning input. It tells you when to watch more closely and when to ask a contractor for an opinion on replacing the unit. A fan that clears the room in good time, check after check, is doing its job, whatever its age.
Keep the log
Keep one page per fan. Record the install year, the clearing time, the sound, the condition of the cover, the exterior flap, and any staining. Add a line each time you check. The changes show up in the log before they show up on the ceiling. Bring the page to whoever you call, so they start from what you saw rather than from memory.