I think the only way to do it is take a picture and then read the EXIF
data on that pic, which includes total exposures taken with that
camera.  It will require software to find it, since most don't read
*all* the EXIF data.  Two are EXIF tool and EXIF viewer (I think) and
they will read the field with 'total exposures in it.  Maybe somebody
else can be more helpful, but I think if you Google those names, you'll
find the software.  Now, how you take a pic and read it is your problem
to work out.  Best case, you have a laptop with a card reader you can
take along and do it on the spot.  Otherwise, you'll have to ferry the
card back and forth before the purchase.

Kitt