Suppose I want to translate "I pinched myself". I see that "to pinch" in French is "pincer" , and that there (more or less) isn't a "se pincer" entry.
I know that a typical pronominal verb, such as "se demander", uses être instead of avoir when in the passé composé. So, "Je me suis demandé" is correct, and "Je m'ai demandé" is incorrect.
So, "I pinch myself" would be translated by "Je me pince". But, since there is no "se pincer" entry in the dictionary, I'm not sure which of these two is correct: "Je me suis pincé", or "Je m'ai pincé"?