My old policies had online handbooks that discussed what was and wasn't covered. The first thing I did when I was looking into top surgery was to find the online handbook and look for the "exclusions" section, which in my experience is always listed in the table of contents. If you can't find the section but the handbook is searchable, you might look for the word "sex" because I often saw the exclusion phrased as "changing of sex" or something similar. But if it's specifically NOT covered, it will likely be in the exclusion section. If you don't find an exclusion, then call.
Most insurance that doesn't cover trans stuff tends to go out of its way to make sure you know about it up front.