Skills tests require the examiner's signature.
When being put forward for a licence or rating, the school will stamp and sign your logbook as being correct.
It's worth getting signatures against important flights like club check-outs, differences training, revalidation requirements etc.
Other than that, it's up to you to be honest. The CAA do occasionally check up on people's experience, and there is plenty of other supporting paperwork to say whether you flew or not. If things smell a bit fishy, I guess that they're more likely to follow up. I have had my overseas experience very prominently signed and stamped, and have the option of a letter from the club's CFI to back it up, just in case.
I even heard a story that the CAA have a Tipp-Ex reading machine. They're going to have fun with my log book, because I have made loads of mistakes that I've Tipp-Exed out
[This message has been edited by foghorn (edited 17 January 2001).]