It wouldn't surprise me if they phoned up airports to verify t/o and landing times once in a while
Yep, this would at least identify flights that never took place. Even here though your takeoff and landing times may be legitimately different from your logged time. Particularly at busy fields where you may have taxied away from your start position, and been held for 20 mins before taking off. There is a lot of scope of fudging your logged hours.
It has certainly come as a surprise to me (as someone who scrupulously records their time I might add!) to realise how few checks and balances there are in the system.
