A cooling system pulls moisture out of the air. That water collects on the indoor coil, drops into a drain pan, and leaves the house through a condensate line. When the line stays open, you never see any of it. When it starts to slow, the first signs are small, and they show up in the same few places each time.
What you are looking at
Stand at the indoor unit with a flashlight. Find the drain pan under the coil. Find the condensate line leaving it. It is usually a small white or gray pipe that runs toward a floor drain, a utility sink, or an outside wall. Many units also have a float switch, a small safety device in the pan or on a secondary pan that cuts power to the cooling when water rises too high. Note which of these you have. The answer changes what each warning sign means.
The signs to log
Write each sign down in your own log with a note of when you saw it. A phone note is fine. A sticky note on the unit is not, because it gets lost.
Water on the floor, or a stain on the ceiling below the air handler, or damp insulation around the pan.
A sour or musty smell at the indoor unit or near the drain outlet. That usually means growth is building up inside the line.
Slime or algae at the point where the line exits the house, or at the drain termination.
Cooling that stops with no fault showing at the thermostat. This can mean the float switch has tripped.
A pan that is damp at the access panel after a cooling run, even when you see no puddle.
What you can check yourself
Turn off power at the indoor disconnect before you open the air handler panel. Look into the pan for standing water, rust, or cracks. Check that the line slopes down away from the pan so water can run out by itself. Go outside during a cooling run and look at the drain end. You should see water dripping there. If you see nothing, the line may be blocked.
Do not force a rod or compressed air into the line unless you know what the fittings are made of. Do not pour harsh chemicals into the pan.
What to hand off
Clearing a line with a vacuum or a flush, cleaning the coil and pan, replacing a cracked pan, testing or repairing a float switch, and adding a trap where the design calls for one are technician work. Some of these may also need a permit, depending on where you live.
What to ask before anyone opens the panel
Ask whether the line has a trap, and where it terminates. Ask whether the pan has a secondary drain or an overflow switch. Ask whether the visit includes a test of the float switch and a check of the line slope. Ask what the cleaning covers and what it leaves out. Ask whether the pan will be inspected for cracks while the panel is open.
Using the log for planning
Keep the install year beside the log. A slow drain you clear once is maintenance. A slow drain that keeps coming back, a cracked pan, or repeated shutoffs in an older unit is a planning input. Those entries tell you the cooling system is nearing the end of its service life, even if the compressor is still running. Check your system's typical service life on the Central Air Conditioner page, and if your unit is a heat pump, the Heat Pump page. Use those figures as the clock, and use your log to decide how close you are to it.