Only the owner of the points can check with Member Services about the reservation.
The person you are renting from should be able to email you a confirmation number if not the entire confirmation. You can link that number up with My Disney Experience and check it that way.
I have rented 3 different times from other owners and have never had an issue.
As noted owner should give you reservation number and you can put that in your MDE account, and if successful in doing so the reservation is thereby confirmed. Once in the MDE, the owner cannot change it without your knowing about it because any change would also appear in your MDE account