Use a program that can pull out Maker's Tags from the EXIF data, such as EXIFTool.
If you're a Mac user, it's easy and doesn't require a special program. Start Preview, open an image you've taken. Select Get Info by pressing Command-I. In the box that appears, click the tab with the circled i. Click the Nikon sub-tab. Look at the data under ShutterCount.