This comes down to good communication, attentiveness, and good listening skills.
There is nothing wrong with asking your partner directly about what they like and what they enjoy doing. But, if the direct approach is not your style, then observe how they spend their free time. Observe what activities or things bring them joy to talk about or do.
And importantly – have an open mind! Sometimes we don’t particularly understand some of the things our significant other enjoys, but it does not mean it is wrong or you are incompatible.