My house was built in 1940 and all original wiring is metal clad / BX cable. With Romex, yellow vs white can tell me wire gauge. Is there any similar tell for MC? I suspect it's all 12 awg but I'd like to double check, only thing I know to do is use calipers to measure diameter of the conductor. Thanks
Take a piece of #12 scrap wire and compare it to what you have. If the wire is the same it is 12, If the wire is larger usually #10 if smaller #14 other than an electric range # 14,12 & 10 will be everything except the range it will be much larger. These are the sizes used in a 1940’s era homes for the most part.
If you have a hard time visually comparing the wires, go buy a standard, cheap light switch or outlet that has a back-stab connection (the insert spring clamp kind, not the screw down kind).
These connectors are normally labeled "14GA ONLY" and are designed to only fit 14GA wire. A strand of 12GA will not fit in the hole. Even with poor lighting this should be a simple, foolproof way to identify 14GA and 12GA wire.