The Oxford American Dictionary does give an entry for the noun "scrutinization" in its entry on the verb "to scrutinize". Note the 'z' instead of the 's'!
Using an 's' is preferred in British English, but you won't find it if you're looking in American dictionaries or using American spell checkers. This is part of the general spelling difference between American and British English, in which American English prefers that verbs end in '-ize' while British English prefers '-ise'. According to Mary Norris's book Confessions of a Comma Queen, you can thank Benjamin Franklin for that bit of trivia, as well as for many other spelling differences between American and British English ('color' vs 'colour', 'theater' vs 'theatre', and so on).
scrutinize | ˈskro͞otnˌīz |
verb [with object]
examine or inspect closely and thoroughly: customers were warned to scrutinize the small print.
scrutinization | ˌskro͞otn-iˈzāSHən | noun
scrutinizer | ˈskro͞otnˌīzər | noun