Hola Dylan,

There is no formula to know when a verb is reflexive or not. Sometimes some verbs are used as non-reflexive and reflexive. They often change meaning. 

We use sentarse to say "to sit" but when the person sits somewhere, for example:

I always sit at the back -> Yo siempre me siento detrás. 

But you can also use "sentar" (non-reflexive) in these contexts:

1. you sit "someone" somewhere. For example: "He sentado a mi madre a mi lado." (I sat my mum by my side.) This is a transitive verb, the object here being the mother.

2. you are saying something suits you. For example: "Ese vestido te sienta bien." (That dress suits you.)

Bear in mind this "te" here is not a reflexive pronoun but an indirect object pronoun.

2. you are saying something doesn't agree with you, as in food. For example: "Los pimientos no me sientan bien." (Peppers don't agree with me [I always get tummy aches when I eat them.]

Again, this "me" is an indirect object pronoun, not reflexive.

I hope this helped.

Saludos,

Inma