What I always say to my students is that the case is not determined by the ending, but by the context and sentence where it is used. In your case, genetive can be used with so called possessive meaning, sth belongs to the famer. Filia agricolae pulchra est. The famer's daughter is beautiful. Dative is used as indirect object in the sentence with some of the "giving verbs" where in english are used the prepositions to or for. Rosam puellae do. I am giving a rose to the girl. Rosam puellae emo. I am buying a rose for the girl. Nominative is always used as the subject of the sentence or as the nominal part of the predict with the verb to be. Puellae in schola sunt. The girls are at school. And the vocative is used when talking to somebody direvtly, there is always a coma after it and the verb must be in the 2nd person sg or pl or imperative. Puellae, cenam paratis. Girls, you are preparing dinner. Puellae, cenam parete! Girls, prepare dinner!

you can look for other clues in the sentence as well as context. If 'agricolae' is nominative plural, there should be a plural verb. If dative sg, you should look to see if there is some sort of 'giving' verb. If genitive, there should already be an obvious nominative. This is the fun part of Latin. examples: nom pl- Agricolae in fundo laborant. (verb is plural-The farmers work on the farm.) dat. sg- Senator pecuniam agricolae dat. (giving verb-The senator gives money to the farmer) gen. sg. Equus agricolae sub arbore est. (The farmer's horse is under the tree) hope this helps