What I do is send a confirmation email to myself and in the subject line I write how many points are required for the reservation. However, this won't help if Marriott requires "proof" of what you booked at. The other thing I've done is made the reservation using points (obviously you need the points in your account for this), which will trigger a confirmation email. Then I go back in and change the reservation to a Points Advance reservation. This would probably work as "proof" since your confirmation email would have the reservation number and the points required.

As a matter of fact, I think that's what I'll start doing all the time now as a "best practices" approach.