I am totally guessing at the answer to your question (so basically, it's worthless, teehee) but my GUESS is this. The angle of one's own reach is limited, so the effects of masturbation on the hymen would likely be limited to a certain angle. In some of the reports, the hymenal area in question is referred to as "at 7 o'clock" or whatever. My guess is that changes observed at certain positions (again, one o'clock, 2 o'clock, etc) are consistent with masturbation, others are consistent with someone else performing the act. make any sense?
A second measure might be that the person performing the act on oneself will know what hurts and what does not hurt, and would stop short of inflicting pain on oneself, and will therefore stop short of inflicitng the sort of injury which leads to "changes". The person performing the act on another is not constrained by this pain response, and may, in fact, be doing the act in order to inflict pain (for their own pleasure orfor corporal punishment, as has been speculated in this case) Therefore, the injury/scar tissue/"changes" may be a different degree in the case of abuse. Make any sense?
To steal a phrase from SuperDave I'm just "spitballing".....