ALL pronominal verbs use "être" as an auxiliary. There is no pronominal verb that uses "avoir". That's the only actual rule that is always true. A pronominal verb is a verb preceded by a reflexive pronoun (me, te, se, nous, vous, se) that matches the subject (je me, tu te, il/elle se, nous nous, vous vous, ils/elles se), like "s'endormir" or "se laver".
A few non pronominal verbs (usually describing a movement) also use "être" but most use "avoir".
That's not a rule, just a tendency. You will have to learn which ones.
Aller, apparaitre, arriver, devenir, entrer, mourir, naitre, partir, rester, tomber, monter, descendre, retourner, sortir, passer and venir are very common so you will quickly remember that they use "être".
The prefix "re" just means "again" so verbs like "revenir", retomber" etc exist and are conjugated the same way than the verbs they derive from.
Décéder, parvenir, provenir and survenir are a little less common. But the last four are derived from "venir" so that's a clue.
Intervenir is as common as the ones right above, but somepeople use "avoir" when it should actually be "être". Avoid making this mistake bu be prepared to hear it.
Demeurer is less common but is conjugated with "avoir" or "être" depending on the way it's used. For a general meaning of "to stay" (rarely used and usually followed by an adjective), it's "être" but for the meaning "to reside", it's "avoir".
Échoir is rare and you will mostly see it in legal documents. People often assume it uses "avoir".
And finally, there are a few verbs that are not used anymore, like débeller. Don't bother learning them.