I have stumbled upon several listicles that list dozens of signs you're a mom. But I have boiled it down to six: 

You pushed a human out of your vagina

You had a human pulled out of your abdomen

You legally adopted a human and you identify as female

The person you are in a committed relationship with did any of the above three things and you identify as female

You and your partner put all or some of your DNA in another person who then delivered the baby and gave it to you and also you identify as female

Somebody calls you "mom" and not by accident