Slow draining
The earliest and most ignored sign, because it happens gradually enough that people adjust to it without noticing.
A healthy sink empties briskly with a clear vortex. If a basin now takes noticeably longer than it did a year ago, material has been accumulating on the pipe wall — and it accumulates faster once it has started, because the narrower channel slows the flow, and slower flow drops more material.
In a bathroom this is almost always hair bound together with soap scum. In a kitchen it is fat and food residue. Both respond well to being dealt with early and badly to being left.
The useful question is whether it is one fixture or several. One is local. Several is not.
A smell that keeps coming back
Two quite different causes, and they are easy to tell apart.
A stale, sour smell from one drain is organic material coating the inside of the pipe just below the outlet — biofilm. It is unpleasant rather than serious, and cleaning the tailpiece and trap deals with it.
A sewer smell — unmistakably sewage rather than staleness — is different, and it means a seal has been lost somewhere. Most often a trap has dried out in a fixture nobody uses, which lets sewer gas up through the empty U-bend. Run water in every unused sink, shower and floor drain for thirty seconds and see if it clears.
If it does not clear, the venting or the line itself has a problem. A drainage system is sealed; you should not be able to smell it.
Worth knowing
Guest bathrooms, basement floor drains and utility sinks are the usual culprits for dried traps, particularly through a dry summer with the air conditioning running. It is worth running water in them monthly for that reason alone.
Gurgling
Gurgling is air being pulled somewhere it should not have to go, and it is often the earliest warning of a main line problem — well before anything backs up.
A drainage system needs air behind the water to let it flow, supplied by vent pipes running up through the roof. When a line is partially blocked, water passing the restriction pulls air through the nearest available opening instead — which is another fixture's trap, and that is the noise.
The detail that matters is where it gurgles. If flushing a toilet makes a nearby tub or basin gurgle, those two fixtures share a restriction downstream of both. That is a main line symptom, not a fixture one.
If a single fixture gurgles only when it drains, its own trap or branch is partly obstructed — a smaller problem.
Clogs that keep coming back
This is the most diagnostic sign on the list, and the one most often mishandled.
A properly cleared drain stays clear. If a line was cleared and blocks again within a few months, the clearing worked — the problem is that something is causing material to catch in the same place repeatedly.
That something is usually structural: a sag holding standing water, a joint that has separated, roots entering through a crack, or a section that has offset as the ground moved. Our clay soil produces all four.
Clearing it a third time is buying the same afternoon over and over. At that point a camera is cheaper than the next two visits, because it establishes whether you have a blockage problem or a pipe problem — which are very different jobs.
More than one fixture at once
The single most useful distinction in drainage, and worth checking before anything else.
Every fixture in the house drains into one main line. A problem in a fixture's own trap or branch affects that fixture alone. A problem in the main affects everything upstream of it.
Symptoms appear first in the lowest and closest fixtures to the main — typically a ground-floor toilet, tub or shower. A downstairs bathroom misbehaving while upstairs is fine is a textbook main line pattern.
The classic confirmation: run the washing machine and watch a nearby shower. If water rises in the shower when the machine discharges, the main line cannot take the volume. That is not a fixture problem and no amount of plunging will help it.
What to do, and what not to
For a single slow fixture, mechanical clearing works: a hair catcher, a plunger, and pulling the trap if you are comfortable doing so.
Do not reach for chemical drain cleaner. It cannot clear roots, a sag or a wipe accumulation — the things that actually cause repeat problems — and on a blocked line it sits in standing water, caustic, waiting for whoever opens the pipe. Repeated use also damages older pipe and rubber seals.
For anything affecting multiple fixtures, or anything that has come back after being cleared, the drain is not the thing that needs attention. The line is.