I started another thread after I had fuel running out of the fuel cap when I gently dropped my Breva 1100
It was caused by the rubber seal not pressing down properly on the seat and courtesy of Nick on here and the lads on the Aprilia forum I found a solution
See here
http://wildguzzi.com/forum/index.php?topic=83090.0However it struck me that most of us wouldn't know we had a problem until
a) you drop the bike like I did
b) you've fallen off and are lying underneath it with fuel running over you ........ which would be BAD!
The appear to be 2 ways of checking the caps.
Fill the tank up and then
1) lay the bike on its side and see if the fuel comes out of the filler cap
2) make like the Aprilia lads and pop a big ole wheelie and see if your genitalia get marinated with fuel

Lets be safe
(as far as you can riding a motorcycle) out there