The main signs that a sewer drain is clogged are water backing up in a drain or toilet and gurgling noises from the drains. Low-lying floor drains in basements are often the first sites of water return, so keeping an eye on these drains can help to identify clogs early.
Slow flow during draining can also be a sign of an impending clog, as can bubbles forming in the toilet bowl once a flush has settled. Any of these signs is worth investigating further with the help of an experienced plumber. Sewage backups, which occur if the clog becomes impacted, are difficult and unpleasant to deal with.