When you are spectating a mob and you press the key to open your inventory (the default key is e on Windows 10) you will see your username where the picture of your body would be if you were in another gamemode (between the off-hand slot and the armor slots) your username will show. (Note: I spectated a horse in the nether over magma blocks and a strider in the nether on lava, but I do not think that it affects this)