Er.... I assume you mean nouns rather than vowels there. There are some general things, but overall you learn them as you go. My advice is to remember them with an adjective that lenites so you can remember which they are. But as a rough guideline, masculine is used for names of males, nouns ending in -(e)an, -(e)as, -air, -ear or -(a)iche. Tends to be names of trees and the young of animals. Feminine is for names of females, nouns ending in -(e)ag, -(e)achd, or -e Names of countries, musical instruments, diseases and planets. But this is just general guidelines and it's best to learn the gender of the noun as you learn it.