Nouns in Latin are categorized in what we call declensions. The declension is determined by the genitive singular of the noun. Most nouns in Latin in the 2nd declension end with -us. This shows that a noun is masculine. However, there is another type of 2nd declension noun that is masculine. They end in -r, but all the other endings are the same. These nouns generally end with -er, but sometimes just -r. In either case, the main difficulty is determining if the -e- stays or drops out. The -r always remains. There are two ways to determine is the -e- stays of drops.
1) If you are unsure if the -e- stays or is dropped, think of an English word that comes from the Latin. If the English word does not have an -e-, the Latin stem (to which you add the endings) does not contain an -e-.
2) As I am sure that you are currently doing, you should memorize the nominative singular, the genitive singular, the gender, and meaning for every noun that we have studied. The stem of every Latin word is based on the genitive singular.
Adjectives are slightly different since you don't have to know the genitive. For adjectives, you can use the English method to determine if the -e- remains, or you can learn the feminine form.