Any tricks for finding a leak in fuel line ?

Back in the 80's a kid got killed in a truck shop I went to. They had a 55 gal barrel of oil and it had a reg on it, guys would plug in the air hose and oil would get pumped out into the can they used to dump it in the motor. Some jerk took the reg and the kid plugged the hose on and it blew the bottom out of the barrel and the lip on top of the barrel hit him under the chin and cut his head in half. He died instantly and the barrel hit the roof of the shop. It put the guy out of business.