That looks like a 10". Some are stamped with the max diameter allowed on the flat surface between the studs. It only takes a few minutes to pop the grease cap off, remove the cotter pin and outer bearing, then slide the whole hub and drum assembly off. If you're going to do a brake job, you're going to need to know whether the drum is still within specs anyway. You'll also want to check the condition of the brake hardware, wheel cylinder, and bearings, so, pull it off and check everything.
