I pick a few files at random and check to see if I can read them from the last few days backups, and that they have the correct mode, owner, etc.
Actually, I have a script that does that for me and just emails me the results. Occasionally it tests a file that is in the backups and no longer on the machine being backed up (or vice versa, or a file that has changed etc) in which case the script moans VERY loudly and I make sure I take the time to check that everything really is OK and it's not an error.
Doing that is sufficient for file backups like what rsnapshot does. People whose backups can be used for a "bare metal" restore, inlcuding things like disk layouts, boot sectors and so on, should test their backups by periodically doing a full restore to their spare hardware.
BTW, I very strongly recommend Curtis Preston's book Backup and Recovery. The link is to a mini-review I wrote.