I bought a CO test meter on Ebay. Great buy. I use it as a sniffer. It is very sensitive. I used it to find leaks in exhaust.
It even help me diagnose a bad head gasket in a 440 once. I had it sniff CO in the radiator. Caught it while the chemical tester was inconclusive.

The meter I bought was a used construction grade level CO tester.
You just have to be careful that it does not get a whiff of the tailpipe. It is very sensitive.