[ARCHIVE ANSWER]

The SCUBA tank and propane cylinders are definitely testable and can be had for pretty cheap.
They're designed to contain gas under high pressure without escaping. Assuming no external contamination, it would seem like the weak point on containers like that would be the welds and the valves. It could be tested by filling a cylinder with
weed, closing it, and then decontaminating the outside thoroughly with rubbing alcohol or solvent and then letting a K-9 have at it.However, filling a tank with weed will make it sound and weigh differently than if it was filled with air and/or
gas. The other thing is that the average person just wants to get their one or two baggies home safely, so a scuba tank isn’t practical. Kops might also get suspicious of why you have a scuba cylinder in your car. If you have some
paintball gear in your car you could just explain you use it to fill the air tanks. Then again, tanks are highly suspicious because meth cooks use them to transport anhydrous ammonia. Anhydrous is nasty stuff that causes severe chemical burns.
A propane tank is not designed to haul other gases.