There are various detection methods ranging from a few bucks (detection paper) to 120,000 USD.
I have extensively used used gear on both ends of this spectrum .
I only stuff I know thats affordable for civilians and still useful is the M256 ( but u ought to be already masked when using it and the concentrations its sensitive to are already in the dangerous range)
PIDs and such which are better however run in the 4 digit cost range.
Your best sensor is your brain though looking for people keeling over = indicator to mask.
If you are the person keeling over you cant mask anyway and a detector would not have helped as they (with few and expensive exceptions) dont operate independently and continuous.
Even in the .mil you use detectors only when indications or warnings exist.
I guess am having difficulty describing my point, but it essentially is, that for the prepper money spent in Gas detectors could be spent elsewhere unless you are a wealthy prepper ( and there are of course a few of those)