How do you know whether to use avoir or être in the perfect tense?

Most verbs form their perfect tense with avoir. There are two main groups of verbs, however, which form their perfect tense with  être :-All reflexive verbs -A group of verbs that are mainly used to talk about movement or a change of some kind
The verbs in the second group can be remembered through acronym DR MRS VANDERTRAMP:
Devenir (to become)Revenir (to come back)
Monter (to go up)Retourner (to return)Sortir (to go out)
Venir (to come)Aller (to go)Naître (to be born)Descendre (to go down)Entrer (to enter)Rentrer (to go home/to return)Tomber (to fall)Rester (to remain)Arriver (to arrive)Mourir (to die)Partir (to leave)
When you are working out which auxiliary verb to use, just ask yourself 'is it reflexive?' and 'is it a Vamdertramp verb?' if the answer is yes to either question, use être and if it's no, use avoir.