rconner,
Engineering is ultimately a matter of expert judgment based on sound (but necessarily complete) evidence.
If you absolutely, positively want to verify the compatibility of HDPE (or any other piping material) with the contained fluid, you would have to demonstrate it with an actual sample of the fluid according to the procedures in the following link.
Most people do not have the time nor the resources to go out and test every application on every project so that you can be assured that you are scientifically correct 100% of the time.
baltmore is talking about a gas station project. That would generally be a small site maybe 150 ft x 150 ft. The piping would probably be mostly 1-2 inches in diameter. The pumped fluid is probably at low operating pressures. The concentration of benzene in gasoline product is only 1-2%, so the concentration in remedial ground water will be much less. 4-5 mg/l of benzene in the ground water would probably be considered to be a high level of contamination. He is probably talking about a short-term project, not something that will go for 20 years. In short, remediation at a gas station would not be considered to be a demanding application.
In light of the above comments, even if the piping material that you selected turned out to be inappropriate, the consequences of the piping decision are not that great. Nobody is going to be injured. The worst case, you probably have to replace some small bore piping that costs about $5 for 10 feet.
On the other hand, if you were selecting a landfill liner that must last indefinitely, you would not want to be so flippant with the liner selection since the consequences of a failure may be extremely costly.