Yes, NM-B (aka Romex) had a white jacket for both 12 and 14 gauge wires a while back (certainly in the 80's and probably early 90's). It's never safe to assume, especially if you think a homeowner or other unqualified person did the work. Find yourself a piece of 12 or 14 gauge wire and compare.

The cable sheath should be marked with printing indicating it is type NM and what the gauge is (the colors as Mike says haven't always been universal). Further, some other wiring types (like UF) don't color code by gauge.

If you can't see the printing, the easiest way is to open up one of the boxes and look at the bare conductors. 12 and 14 are pretty easy to discern at that point.

thanks. I will look closer and will get a sample of both gauges so I can compare.

NM cable sheath slitters are available which also have measured holes on the side that indicate the gauge of the wire...14 to 6. The difference between 14 and 12 gauge is quite apparent.

I couldn't detect any difference in the wire. I'm wondering if that yellow cable fades over time. So thanks for that.

Can you not find any printing/embossing on the sheath anywhere?

Not enought sheathing sticking out at the switches or outlets. The one run I can see I can't get close enough for a good look. The only thing I can do is compare the wire. It is difficult to work, more so then I recall any time I've put 14 gauge wire in. That also makes me think it is 12 gauge.

The color coding just came in a decade or so ago. I just finished up using the last of a spool of white 14 ga NM cable, still have a bunch of white 10 ga NM on hand. Just measure it to determine gauge. 12 is about 0.08", 14 about 0.065".

thanks for those measurements.

get a wire stripper, put the bare wire in the gauge sizes and size it up :P (kill power first).

people that play with wire often can tell the difference, 14 gauge is easier to work with due to it being thinner. longer you play with it you will be able to tell just by looking at the bare wire thickness.

otherwise, no you cant tell on old wire without reading the marking on the jacket. pretty much the only way you can go by jacket color is if you know it is new wire.

Ok, thanks Vith.