My car'* A/C has 134a , can I top it up with 12a refrigerant.

Is there a way to tell if refrigerant is low without a gauge. Can you measure the outside temp and temp coming from a/c vents and if the difference is certain degrees then refrigerant is likely OK ?

No. It'* amazing that you actually have access to R-12 . They can't be mixed and the respective air conditioning systems are designed for the respective refrigerants.

No, unless it is so low that it doesn't cool, and even then, the symptom "doesn't cool" could be a number of things besides low refrigerant.

This is a poor way to tell the refrigerant level accurately.

I have a can of "Ultra Cool A/C on the GO" . It is 12A refrigerant W/dye. It says that it is Synthetic 12a Replaces R-134a lost refrigerant & oil to the a/c system ? I have attached pic

To be charging the syst4em without knowing the state of charge/pressure is asking for trouble. Use that stuff when you know you have a leak. You don't know if you have a leak.
Look at the connections. Any oily grime around the connection? That'* a leak. Look for signs of oil at the compressor.

I have "arctic freeze" with gauge. It was for 134a . The store person told me I can use the gauge on this to measure if I am low and can use it on this 12a can to top up.

Okay, it looks like I have to eat my words above.

I thought you were talking about plain old R-12. I didn't know about R-12A.

It looks like a mostly a good thing if the construction and performance can be believed. I've read that it isn't allowed in public transport vehicles due to it'* extreme flammability.

All of that having been said: I don't see evidence of many bazillions of people using R-12A for years and decades and reporting back on success or failure. I would think of this as an experiment. If you really want to try this on your car, I'd be interested how it goes.

Back to your car: You can also overcharge your refrigerant. Cooling happens due to pressure changes through the different parts of the cooling system. If you overcharge it, that will make it not cool as well, and likely damage things.

I suggest that it is important to actually diagnose whatever issue it is having before blindly adding refrigerant, experimental or not.

I wrote to the company that makes the R12a (ultra Cool) and this is the response I got (attached). Princess Auto seems to sell lots of this product and looked like they had sold lots from the end shelf. I am not sure if mine has a problem, My wife said that it doesn't cool as much as previous but that is subjective and I will confirm with pressure gauge.

Cool!

Good plan!