I leak check with a sniffer (get close), soap bubbles (narrow it down), or UV dye kit for the super small leaks (months). Finding a leak visually can be near impossible, unless it is a huge leak.
I have worked in a few companies that use R414A or R414B as direct replacement for both R12 and R134. That means either gas can replace BOTH R12 and R134A. It seems to work fine. I have also used them in new R134A systems without any issues.
These gases are nice because you only have to carry one tank of refrigerant for all cases.