The signs, in the order you should trust them
Not all of them mean the same thing. These are sorted by how much weight to put on one, starting with the sign that is almost never anything else.
Two cycles for one load. The most reliable sign there is. A dryer works by moving heated air through the clothes and out of the house. When the duct is packed, that air has nowhere to go, the drum fills with humid air it cannot expel, and the machine runs and runs without drying. If this started gradually over months, it is the duct.
The dryer or the clothes are hot to the touch at the end. Heat that should have left with the air stays in the drum instead. A load that comes out hot and still damp is the same restriction showing up a second way.
The laundry room gets warm and humid while the dryer runs. Air that cannot get out through the duct leaks back into the room, and it brings the moisture with it.
A burning or musty smell. Burning means lint sitting against something hot, and it is the one sign on this list that is worth stopping the machine for. Musty usually means moisture trapped in the duct, which is its own problem and points the same direction.
Lint around the door seal or behind the machine. Lint pushed back into the room instead of out of the house. Worth noticing, less reliable on its own, because a torn transition hose does this too.
Nothing coming out of the vent hood outside. Covered in the checks below, and it is the one anybody can do in a minute.
Three checks you can do yourself, in about ten minutes
Stand at the outside vent while the dryer runs. Put the machine on a heated cycle, walk outside, and find the hood or the roof cap. You should feel a strong, steady push of warm air, enough to be obvious with the back of your hand. Weak, intermittent, or nothing at all is a restriction. If the flap is not opening, that alone can be the whole problem.
Time a normal load. Not a towel load, not a comforter. A normal mixed load on a normal setting, timed against what that same load used to take in this house. The comparison is the test, not any particular number: a load that has quietly gone from one cycle to two, with a clean lint trap, is the duct rather than the dryer.
Pull the machine out and look at the transition hose. That is the flexible piece running from the back of the dryer to the wall. Crushed flat behind the machine, kinked into a tight bend, or packed with lint are all common, all visible, and all fixable in the time it takes to look. This is also the section you can safely clean yourself.
What none of these three reach is the run inside the wall, the ceiling or the roof, which is where a real blockage lives. That part needs a brush on a rod and a vacuum holding suction at the other end.
Why a clean lint trap proves nothing
The trap catches a fraction of what the dryer sheds. The rest goes past it and into the duct, which is exactly what the duct is for, and it only becomes a problem when enough of it settles at a bend or a screen to narrow the passage.
So a spotless trap and a slow dryer are not a contradiction. They are the normal presentation of a duct that needs clearing, and it is the single most common reason people conclude the machine is dying.
The other thing worth knowing about the trap: pulling it out and vacuuming the slot it sits in is genuinely useful between visits. It is just not the same job.
When it stops being a laundry problem
Lint is fuel. A dryer is a heat source. A duct packed with lint puts a large quantity of the first directly against the second, and the mechanism is that simple. It does not need a statistic to be worth taking seriously, and the sign that matters is the burning smell.
The slower cost is the machine. A dryer fighting a blocked duct runs longer for the same result, which is both a power bill and wear on parts that were not designed to run double cycles for years.
A nest is the other case. Birds get in through a flap that stopped closing, and that blocks a duct completely rather than gradually. The signs of a nest are different from the signs of lint, and the difference is easy to hear.
What you can clear yourself, and what you cannot
Yours to do: the lint trap and its slot, the transition hose behind the machine, and the first foot or two of the duct at the wall. That covers real lint and takes twenty minutes.
Not yours to do: the run itself. A shop vac does not reach it, and a leaf blower is worse than doing nothing, because it pushes lint deeper and packs it against the first bend. That is the opposite of the goal, and it turns a restriction into a plug.
A roof cap is its own case. It has to be cleaned from the roof, which is a ladder and a vertical run of up to twenty-five feet with a vacuum at the dryer end.
What a visit settles
Airflow gets measured before anything is touched, so there is a number rather than an opinion about how restricted the duct was. A rotating brush runs the full length with the vacuum at the source, the hood or cap gets checked for a flap that actually opens, and airflow is measured again.
You get both readings and photos of what came out. If the duct turns out to be fine and the problem is the machine, that is what you hear. The main page covers what a cleaning involves across the valley, and the Henderson page covers upstairs laundry rooms and roof runs.