Fume events are usually described as smelling like dirty socks or a wet dog. There was a fume event a few years ago that ended up going very badly for one of the pilots:

http://avherald.com/h?article=4b6eb830&opt=1024

The fuel smell at the beginning of the flight might just be happening because the engines and/or APU are ingesting the exhaust of the aircraft in front of you while on the taxiway, and some of that exhaust smell ends up in the bleed air that eventually enters the cabin.