If you have Windows, open up a file with Opanda (Google that) and it'll tell you on a line somewhere the number of shutter activations (actuations). For Mac OS X, you can actually see it in one of the info screens of the preview program. Or this guy here has a pretty good EXIF viewer.
Shutter Actuations for D100
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Aaron Chong
2006-09-29 09:34
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Can the program determine the shutter actuations for a D100?
Opanda
2006-09-29 10:37
Opand IExif only support D200/D2H/D2X/D70(s)/D50(s) and later.