The drain is slower than it used to be
Buildup narrows a drain gradually. A sink that used to empty in seconds and now takes a minute is telling you the pipe is mostly closed. That is the cheapest time to clear it.
Water stands in the tub while you shower
Hair and soap scum in the tub drain or trap. This one you can often clear yourself: see how to unclog a shower drain.
Gurgling sounds
Air pulled through a partly blocked drain or a vent problem makes a gurgle. A gurgling toilet when another fixture drains points further down the line. More in why is my sink gurgling.
Smells that come back
Food, grease, and biofilm coating the inside of a drain smell even when the water still drains. If cleaning the stopper and trap does not help, the buildup is further down.
More than one drain is slow
This is the sign not to wait on. Several slow drains, or water coming up in one drain when another is used, points to the main line. See how to tell if your main line is clogged.
The same drain clogs over and over
A drain that needs clearing every few months has a reason: grease buildup in a kitchen line, roots, a belly where water sits, or a pipe with too little slope. Clearing it again without finding out why means paying for the same call again.
What to skip
Chemical drain openers rarely clear a real blockage, can damage older pipes, and make the job more dangerous for whoever opens the drain next. More in chemical drain cleaner risks.