The Firefox extension includes both versions, so I see a red underline for advertize (usage is archaic), but not for either realize or realise. aspell -d en_

The Firefox extension includes both versions, so I see a red underline for advertize (usage is archaic), but not for either realize or realise. aspell -d en_GB-ize-w_accents list < file-to-check is the command to merely list spelling errors.