I was foolish enough to have some AC work done by what turned out to be a rather sloppy outfit - is there now any easy and reliable way
for me to tell if my AC system has R12, R134a, a mix, or something else in it without lots of expensive equipment?

No. Just drive up to an A/C shop, tell them you think some work was done improperly, and ask if they'll please "sniff" your A/C system to see what refrigerant was installed. They'll probably do it for free or cheaply.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Ha ha ha ha ha! Yeah, rite! Not in these parts, at least...

D/W

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If it ain't broke, fix it!

It is possible to read the static pressure and compare the system pressure to a pressure/temperature chart, but that is not totaly reliable.
Bring it down here and I will do a snif test for free

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Thanks Charrlie_S for the offer of a free sniff; I'm afraid the distance (from VA) makes it impractical. It looks very difficult to do a vapor/pressure identification; there are too many variables I can't control easily.

I called a reputable local AC shop to ask about getting it sniffed; they could do it for $48 about 2 weeks from now. Would they be legally required to turn me in to the EPA if the system is contaminated?

From the MDS sheets it seems R12 (CF2CL2) can't burn, but R134a (CF3CFH2) can; but it also looks like both mineral oil and PAG oil can burn. Both R12 and R134a produce really nasty stuff when heated enough.

Does that mean that the AC system represents a fire or other health hazard do the passengers if one of the old lines under the hood suddenly ruptures? Is there any online documentation on the subject? Would trying to ignite a small sample to convince myself it isn't a hazard be a really bad idea?

Caramba! Keep letting your fingers do the walking. Many shops around here advertise a free A/C system inspection.

No, and there's nothing they could "get" you on even if there were such a "must turn in" law.
All correct.
The answer is "potentially and theoretically yes" with ANY refrigerant, but in practical, real-world terms, the danger you describe is minimal, for there's enough underhood airflow and enough sealing between underhood and the passenger compartment that a ruptured underhood hose will tend to dump the system into the atmosphere rather than into the car. This happened to me in the middle of the San Joaquin desert w/R12 in my '65. All that happened was that I heard a "PFFASHHHHH!", smelled refrigerant oil, and started sweating.
EXTREMELY bad. Don't do it.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

in theory R-134a can be explosive IF mixed with high pressure air. R-12 on the other hand, when exposed to an open flame, will make phosgene gas, which is poisonous. Even if r-12 is drawn into the carb of a running engine, the gas comes out the tail pipe. There are other refrigerants sold for auto A/C, which contain propane and/or butane. Keep away from these.
Don't worry about a fire hazzard, when using approved refrigerants. Even steel will burn under the right conditions.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225