They will have checked and repaired themselves. It can tell, because it leaves a flag for itself to check whether the filesystem was unmounted safely or not. Your system would refuse to boot if it couldn't repair them.

The damage isn't completely random. It happens when a filesystem operation was left half-finished because of a power-failure or other crash. This means that, if anything happened, things(files or folders) which were being often modified are the most likely to be damaged, truncated, or missing.

This also means that a partition which almost never gets written to is very unlikely to be damaged by a power outage. This is one good reason to split up your system into partitions instead of lumping everything into /...