Maybe the easiest, what i did was a linux script.
It create files in a predictable way, some with odd properties, like : their owner doesn’t exists.
Then it backup them and restore them, and check if the restore is the same than expected.
Urbackup shouldn’t crash on anything, not should any other program.
if it does, then just report that.
And restored files should be like you backed them up (urbackup restore symlink as file for example).
If you know well osx or windows, you can try rewritting that for it. For example there has been issues with osx sparse files, and not enough users test their restores.