The first answer is that
I tried using a highly precise single-frame FoV measuring technique, got something slightly greater than 90, but then had to consider that the subtle barrel distortion filter would skew the result very slightly high.
I really need to get around to doing a writeup on FoV measurement techniques so that I can be less cryptic...
The second answer is that you can get a somewhat rougher measurement via a more intuitive approach: Go to a spot where you're surrounded by highly-distant stuff*, level the camera at the horizon**, and do a full 360-degree rotation at constant angular velocity. Divide the time taken for the camera to pass one frame worth of stuff by the time it takes to do a full 360-degree rotation, and multiply the result by 360 to put the answer in degrees.
Here's a quick demo video I scraped together, where I'm doing some constant-speed rotation from 33 seconds to 85 seconds in the video. A full rotation takes around 36 seconds, and if you look at a skybox object on the left side of the image and measure how long it takes to get to the right side, it's about 9 seconds. (9/36)*360 -> 90.
//==============================
*Highly-distant stuff doesn't parallax much with third-person camera rotations, so its movement across the screen when you rotate is "purely" a matter of your rotation speed. For this method you need at least one frame in your rotation to have a highly-distant reference detail on both sides of the frame.
**In rectilinear perspective projection, this ensures that horizontal position on-screen isn't dependent on the y height of a detail in the game world. The already-mentioned barrel distortion breaks this somewhat, but only to the significance of a few pixels on the left-right sides of the screen, not enough to muddy the measurement all
that much.