So what? Some dating-but-not-married-nor-engaged couples wear such jewellery too. EDIT: and if you forgo the ''is higher-status than you'' part (which for certain values of ''status'' would mean you come off as a gold-digger) you don't even need it to be very expensive, and even a picture of the two of you kissing as the wallpaper on your mobile phone would suffice. (If they know you know they know you have a boyfriend --even if he's not higher-status than them-- and they hit on you anyway, they lose plausible deniability all the same.) EDIT 2: On reading chaosmosis again, I realize it's a male and he's asking about how to find out if a woman has a boyfriend. If so, the answer is ''You ask them.''