In Korea, most names are given using Sino-Korean words. When we give a female’s name, we have traditionally preferred the Sino-Korean words which mean beautiful, clean, clever, faithful or calm, etc. So, it can be one of the criteria that you can use to assume whether it is female’s name or not. However, you still have to learn a lot of Sino-Korean words and therefore, it would be extremely difficult for you to tell it.

A boy's name sounds masculine while a girl's name feminine, I guess? There's no strict rule, just the feeling of the sound is all. Usually girls' names tend to use ㄹ and ㅇ more frenquently, and boys' names tend to use stop consonants. But it's no way near definite and you shouldn't really count on such. Once again, there is no clear cut between a boy's name and a girl's name. If you think a name sounds cute and pretty, it'd probably be girl's. If it sounds strong or harsh, it'd probably be boy's. Being a foreigner, you might find it difficult to feel out, but that's pretty much the only way, I think.

Good question! I'd like to know it too!