He got a generic tester like this one (I'm having trouble with that link; see below*), but paid $10 more. There are several similar out there. They use 3 AAA batteries which are such a pain how fast they lose power compared to 2 AAs so carry extras. That model cannot be expected to give accurate readings in the 1/1,000,000 units, but it can keep you from getting hurt. If it reads
any CO at all, reject the tank. The sensor will drift in time so date the unit in writing and replace it annually.
* Go to Amazon and search for
Handheld Carbon Monoxide Meter,High Precision CO Detector,LCD CO Gas Leak Detector,Continuous CO Gas Tester Monitor for $30. Looks like this...