My VR18-200mm also has some dust on the (backside of the) front-lens after less than a year. But I'm happily shooting with it.
Only test you can do is at the shortest focal length (i.e. 18mm) and the smallest aperture (i.e. f16/f22) shoot a blank sheet of paper and see if you can identify any spots. Problem with this method: the spots could easily be dust on your sensor.
The other situation, where you might see an effect is (again at wide-angle) shooting something dark lieka row of dark trees against the light and the sun is slightly shining at the front-lens. Then the dust might give some bright spots (in contrast to any dark spots from dust on the sensor). This test is unfortunately also not 100% proof, as light shining on the front-lense might cause any number of reflections (see here
So you're a bit in a bind.
My opinion: You could be perfectly happy with this lens - and some dust will always creep into even a brand-spanking new lens directly from the factory after some time. But if you want mint condition, never buy used equipment on eBay...
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
, My Pictures
, My Photography Blog