For unknown sex traits you discover those during a date. Each date you can ask once for them to tell you what they do/don't like. But they'll refuse to answer if you don't have them maxed out from praise/flirt/kiss/whatever before you ask.
My date routine goes like this: Take them to the garden (random chance to raise their like/lust bar each turn) then Praise, flirt until the bar is 2/3 or 3/4 full, Kiss, take them to the bedroom, Propose if you haven't had sex with them before, then Intimate Talk to unlock an unknown trait if she has one. Then train physic/wit/charm depending on what they or you need to build.