Most Helpful Girl
Women don't always get upset when you notice how they look. It depends what else is going on, though. We're assuming you mean strangers passing eah other in the street, not work colleagues or anything tricky like that:
-- If a random old man decides to leer at you as you walk, that's not flattering, that's insulting.
-- If a random guy smiles or nods at you in greeting while you pass, maybe says hi, that's flattering.
-- If a random guy turns to stare at you as you pass, you're not likely to see his jaw dangling out of its sockets anyway, so it doesn't really matter.
-- If a random guy stops dead in his tracks to give you the once over, depending on the guy, it can be confusing, irritating, or complimentary.
-- If a strange guy gets touchy-feely, that's generally time to call the cops.
If you're talking about someone you have a history with (colleague, subordinate, girlfriend, etc.) then the rules and responses change a little due to extenuating circumstances, office/company policies, etc.