Hola Joe

There is no rule to know what specific ending a word has. It's necessary to learn the word. It's the same in English: why is it "teacher" but it's "accountant" (different endings)?

Once you learn what the base (masculine singular form) of the profession is, then we can apply some rules. For example, if you know that for a "cook" you say "cocinero" and ends in -o, then the general rule is to make it feminine using -a: 

cocinero (male cook) 

cocinera (female cook) 

If the profession ends in -or for masculine, then you generally make the feminine with -ora:

profesor, profesora

vendedor, vendedora...

And other rules that apply for different adjectives. Some of them stay the same.

Here's a basic list of professions if you'd like to have a look.

Saludos cordiales