Nikon is one of the easiest brands to check shutter count because Nikon cameras embed their shutter count data into EXIF fields. Aside from the online method I just mentioned, you can simply search for the “Shutter Count” field in your EXIF viewer, or use the following command in ExifTool...
but I cannot change the internal shutter speed on a Nikon to numbers between 20 and 25, unless I use a remote shutter release. The solution is just to pick one or the other; you should not see too much trailing at 25 seconds, but you can go down to 20 seconds...