Spotting Hint 1 - Turn the print upside down, stops you looking at the picture and concentrate on the tones.
Spotting Hint 2 - Don't over-do it. Creep up to invisability. Knocking the 'white' off a spot is usually enough to hide it from a resonable viewing distance (i.e. hand held). Over spotted 'donuts' stand out like the proverbial dogs bollocks!