The easiest way to see if there are combustion gasses in the water is to go along to a mot place etc, then get them to place the sniffer in the cap opening(not the coolant!!) as you remove the cap, this will pick up any gasses present from combustion.
I always run the YBO 611 head gasket, and if using ARP stud and nut kit, you must torque, then wait at least 4 hours, then retorque again, I do this in sequence starting at 60lbft up 20lbft at a time to finish at 120lbft.
Lots of other people do it different.
Litmus paper tests for acidity not gasses.
tabetha