Shoot a picture, bring the picture on the display and scroll the data pages. On one of the capture data pages the firmware version is listed. If the firmware is 1.01 or 1.10 the you don't have the buffer upgrade. If the firmware is 5.xx then you do.
By the way, if you don't have the buffer and have the older 1.01 FW, I recommend having it upgraded by Nikon. It is worth it for the increased write speed alone. I got it done by Nikon Finland free of charge, I only had to pay the postage a month ago when I got mine.
I think I am a bit Luddite... I also have a D2x, a D3x and a D40x... there's a pattern.