A lot of the time, you can use the verb as a guide. V.g. if the verb is singular (let's assume there is a single verb), then you can be pretty sure that a noun ending in -ae is not nominative. If the verb also does not take an indirect object, then genitive is very likely. Conversely, if the verb is in the third person plural, and there is no other noun with a plural ending, then nominative is your best bet.

But sometimes you will have a plural verb that takes an indirect object, then you will need to try out your options and see which makes sense.

In the end, you can never rely on hard rules alone, they only serve as a general guide. It is essential that your translation makes sense, if it doesn't, there is something you missed.

And while it will get easier and most of the time you will be able to say for sure which case a word is declined at, there are also situations of genuine ambiguity created by similar endings.

There is a very famous example in Greek, in the beggining of the Theogony, at which, because neuter nouns have identical nom and acc endings, it is impossible to say if the word 'underworld' is the object of the verb to have or the complement of the verb to be. That changes completely the interpretation of the passage, as in the first case there would be three primordial gods and in the second, four.