Personally? I consider the 'Sith eyes' to be determined on how far along someone is along the path to the dark side. Emperor Palpatine is pretty much the undisputed Evil ******* of the Star Wars mythos, virtually unparalleled in his wicked ways. Though few consider him so, Darth Maul has been stated by numerous sources to have been "an instrument of pure hatred" -- so while he isn't a psychological mastermind such as Darth Sidious or a zealous brute like Darth Malak, he is irredeemably evil.
Anakin and Dooku are great cases. I don't consider either one of them to be "pure evil". Anakin, at that point, had become twisted and corrupted by the dark side, essentially off his rocker. Whereas Dooku seemed to be in the dark side more or less for the knowledge for knowledge's sake. Not that Dooku is any less evil; he's committed numerous crimes and willingly perpetuated a galactic conflict at the behest of his master that cost the lives of billions. But I don't think that he's either pure evil such as the Emperor, nor is he a twisted lunatic like Vader.