I, being the brave and true risk manager who goes where angels fear to tread, will handle this. Here are a few ideas for you to try:
A US size 12 waist generally runs 30-32 inches. As mentioned, it can vary by manufacturer, but this is a good average.
The next time she is in the shower and cannot watch what you are doing, grab a pair of her favorite pants that use a belt. Thread a fabric tape measure around through the belt loops and see what measurement you end up with. If her pants don't have belt loops, run the tape measure around the waist at the approximate height of where a belt would go. If you don't have a fabric tape measure (a metal measuring tape is not flexible enough for this) run a piece of cord (detcord for the APS crowd) through the belt loops and cut the cord just at the point where it overlaps the beginning end of the cord centered at the front of the pants (where the top button goes). Measure the cord and you have the waist circumference in inches. Correlate the measurement in inches to either the belt size in inches or with how the belt manufacturer lists S-M-L sizes.
Finally, if she already has belts, look to see if there is a size or inch measurement printed on the underside of the belt, usually either under the buckle end, but sometimes at the tip of the belt.
I have bought clothing for women for decades and I have done alright. Except for losing my hair in my early 20's, but I am not drawing a cause and effect here.