well you can use the --checksum option but that involves.... well... a checksum being calculated for each file. by default rsync will copy on different times and sizes, the only other thing to go on is the contents of the file itself, which would most easily be a checksum.
Actually though, maybe you want this, for a looser perspective...
Normally rsync will skip any files that are already the same length and have the same time-stamp. With
the --size-only option files will be skipped if they have the same size, regardless of timestamp. This
is useful when starting to use rsync after using another mirroring system which may not preserve
Be very aware of the danger that you're opening yourself up to there though.