My girlfriend told me that her friend invited her to a movie date with her boyfriend and her boyfriends friend and she asked me what's my opinion on it? how can I clarify if she's not going on a date with the boyfriends friend?

You need to balance your relationship with trust. Trust is one of the very crucial factors in a relationship. I can not tell you how you can reach the trust. Details of the behaviors and expressions are different person to person and so it makes it difficult to guide you without knowing many details that is related between you two and your principles and attitude.

However, the communication is the true key that works for everyone. You asked this question. Means however you say you might trust her. Yet it's a 50/50 trust! You need to rise the trust level between you two. And that happens by a lot of expressions and communications. So you must express yourself and talk to her nicely about the issue and how you feel. Reach a mutual agreement.

Don't forget the fact that she could not tell you this either! It's something important to be aware of. She told you about the event and asked for your opinion. So even if her friends setting up a date, yet her heart is with you. Because she cared enough to ask your opinion and shared the event with you.

Well, why aren't you invited? Sounds like a double date to me.  Could be innocent fun as well but from what you listed I would say that's a double date.