This is a problem that usually occurs in male cats (it is rare in females) due to the blockage of the urethral tube by an accumulation of crystals there, owing to a possibly pre-detected but maltreated bladder stones condition.

The main sign to watch for is the cat repeadly (say 2+ times) trying to urinate without any result (no urine coming out). The cat may try to urinate in the litterbox but it is also possible that it tries to urinate somewhere else (when affected by this condition); therefore you will need to keep an eye out, for such behavior, at all times, especially if you know your cat suffers from bladder stones.