If you aren't sure if you subvocalize while reading, try forcing yourself to imagine the words being read in a specific way - possibly in your friend's voice, or read in a certain easily stereotyped accent. Once you do that, you can see how different that feels from the reading you normally do.

When I try "reading in a Russian accent", my reading speed severely decreases, and the feeling is considerably more auditory than when I am reading with no gimmicks.