Look at the noun, feminine nouns usually end in an e, so you know the adjectives will be feminine too. :
garçon means boy and is masculine (no e at the end) fille means girl and is feminine (notice the e at the end)
Les grands garçons (The big boys) Les grandes filles (The big girls)
Notice how you use the masculine version of big (grand) when talking about boys, and the feminine version (grande) when talking about girls.
I would be very careful with this "rule" : a noun ending by an 'e' is probably feminine... See un père, un frère, un crime, un sucre, un verbe, un livre and see also a lot of noun ending by -age and -isme : un orage, un ouvrage, un nuage, un mirage ; (BUT une image) un traumatisme, un parallélisme.