Testing for identical content (whether using rsync or something else) will not give you a 100% sure answer. Even if the content is the same, the directories do not have to, for instance:

    $ cp -a dir1 dir2
[download]

will create a second directory with identical content, but dir1 and dir2 are different. You'd have to look at inode change time to see a difference, but even then, it's not a garantee.

OTOH, even if you think the content is different, the directories may be the same. If the directories are the same, you'll be looking at the same directory twice, but since you cannot garantee you'll be looking twice at the same time, the directory, or any of its files inside it, may have changed between your two inspections.

Having said that, I don't think there's solution that will always work anywhere. But usually you will know a bit of the environment. If you know your filesystems are local, and no device is mounted twice, checking inode and device number will do. If you know the directories to be tested aren't being modified while you make your check, checking for content may work.