Well, in Spain we go to RAE, to the UN, and to the Constitución 
As to what constitutes a “right”, related definitions of “derecho”:
2. adj. Justo, legítimo. = fair, legitimate
9. m. Facultad del ser humano para hacer legítimamente lo que conduce a los fines de su vida. = Legitimate ability of a human being to perform those actions which lead to his/her life’s goals.
10. m. Facultad de hacer o exigir todo aquello que la ley o la autoridad establece en nuestro favor, o que el dueño de una cosa nos permite en ella. = Ability to do or claim whatever law or authority establish for our benefit, or which the owner or something allows us to use it for.
11. m. Consecuencia natural del estado de una persona, o de sus relaciones con respecto a otras. (El derecho del padre Los derechos de la amistad) = Natural consequence of someone’s state, or of their relationship to others (A father’s rights, the rights of friends)
12. m. Acción que se tiene sobre una persona o sobre una cosa. = Action which can be performed on a person or object.
13. m. Justicia, razón. = Justice, correctness.
14. m. Conjunto de principios y normas, expresivos de una idea de justicia y de orden, que regulan las relaciones humanas en toda sociedad y cuya observancia puede ser impuesta de manera coactiva.= Legal framework.
15. m. Ciencia que estudia estos principios y preceptos.= The study of the legal framework.
~ adquirido: El creado al amparo de una legislación y que merece respeto de las posteriores. =That which, having been granted or obtained under a specific legal system, must be preserved later.
For specific rights, as I said, there’s the UN Declarations and the Spanish Constitution.
One of the rights in the Consti had to be clarified by the Constitutional Court, specifically the right to non-discrimination, where the CC determined that the items listed for this right in the Constitution were examples and not (as in the US) the only cases that count. Specifically, the suit had been brought about discrimination for sexual orientation (not listed in the Constitution), but the CC said that discriminating for anything other than “ability, willingness and actual performance” is forbidden, period. Note that this suit was close enough to the approval of the Constitution that the CC was able to consult with most of the Fathers of the Constitution, who basically said “heck, we hadn’t listed that because we didn’t think about it, but then, we hadn’t listed ‘thee shalt not discriminate against someone for being a fan of Athletic’ either and that would be equally wrong”.
The notion of derecho adquirido is very important: for example, PP had been against same-sex marriage and they did, as promised, bring a suit about its legality to the CC, but something they made clear from the start was that if the CC said it SSM was proper according to the Constitution (and nobody expected them to say it wasn’t), they would not, in a million years, write a law taking it off the books. Specifically, the answer of PP’s president to the reporters asking whether they would create such a law next time they’re in power could be best translated as “do you think I’m on drugs or what?”