One way that a lot of people estimate their number of R-D 1 shots is simply that if you have the sequential file number option turned on, then the most recent file name shows the number of shots the camera has taken.
It's not a 100% accurate guide (especially when evaluating a used camera) since you can't know for a fact that the previous user had kept sequential numbering turned on since new. Also, if you leave a battery out of the camera long enough to drain the backup power source (I think this is a capacitor rather than a separate battery) the camera will "forget" its previous frame count, as well as date, time, and other user-set information.
Still, if it's your own camera and you know you've had sequential numbering turned on since new and have always kept a battery in the camera, it can provide you with a fairly good estimate.