You won't see the sensor as the mirror is in the way and I doubt if you would see the dust anyway.
Take a pic of the sky or a plain white wall at f22 and that will show the dust bunnies up.
Is this the D40 you were looking at?
Heather
Adam, you might like to read up BEFORE touching anything!
Heather
Try a test at highest F stop looking at bright sky or something, this should show the speck almost in focus,
It looks like a short fibre rather than the classic round grey spot which I think is often pollen at this time of year.
If so it might blow off easily but there's a remote chance it's some seriously sticky oil spot or something which when buying you need to take into account.
don't think i dare touch anything to scared of breaking anything