There are a lot of answers to that. If the piping is properly configured (not much "old" pipe is) then running a smart pig is the best and fastest way to find leaks and near-leaks.
If you can't run a smart pig, then if you suspect a leak (from a material balance for example) then you have to patrol the pipeline looking for lakes that didn't used to be there or unusual growth patterns in the vegetation (outgassing from a leak will kill some plants and act as a fertilizer for others).
For oil lines, many people have good luck patrolling the lines from the air. For gas lines, the success is much more limited.
