Uhm…it's rather obvious if you've ever been in Safe Mode. Right up there dead center in the menu bar it says "Exit Safe Mode". Any wallpaper you may have is not there and there's a nice message after unlocking that tells you that Cydia has protected you (although to be honest you may not see that if you actually boot into Safe Mode).
Also, what happens if Apple restarts your phone when you aren't there with them, such as when they take it into the back to be examined?
Yeah, all the way around, better to restore before taking it in.