When do you know when you're supposed to add the female marker at the end of the past participle.

To know if we have to add the female marker, we have to ask ourselves this question: is the auxialiary "to be" (être) or "to have" (avoir). 

If the auxialiary is to have, you don't had any female marker at the past participle. 
Ex: Elle a reçu un gateau. (She has received a cake) -> no marker, because "a" is the auxiliary "to have", avoir.

If the auxiliary is to be, you have to had a female marker only if the actor is a female. 
Ex: Elle est tombée. (She fell down). -> marker because the actor is a female (elle), and the auxiliary is to be (être)
       Il est tombé. ( He fell down). -> no marker because the actor is a male.