How do you know when a noun is masculine or feminine?

Generally speaking, nouns that end in 'o' are masculine (el niño, el perro) and those that end in 'a' (la piscina, la manzana) are feminine. There are other clues in the endings as well. (there are some exceptions, such as: la mano, la foto, la moto)Nouns that end in '-dad', 'tad' and 'tud' are feminine e.g la comunidad, la responsibilidad, la amistad, la libertad, la juventud, la virtud.Other feminine endings are '-ción', 'sión' and 'gión' (la religión, la irritación, la canción, la profesión)