No, I probably can't prove she is an honest character, but I can use quotes to help defend the opinion that she is an honest character.
The following quote I think is the strongest evidence from Clarisse herself that she is being honest with Montag.
"I'm antisocial, they say. I don't mix. It's so strange. I'm very social indeed. It all depends on what you mean by social, doesn't it? Social to me means talking to you about things like this."
The reason I think it shows honesty is because it is self-deprecating. In my experience, if people are going to lie about themselves, they usually try to make themselves look better than they actually are. Perhaps it's a bit like bragging. It takes an honest person to talk about their faults to other people. It takes an especially honest person to talk about their faults to someone that they don't know that well. Clarisse and Montag know each other, yes, but they are not super close friends.
Or this quote:
"You're one of the few who put up with me. That's why I think it's so strange you're a fireman, it just doesn't seem right for you, somehow."
The reader knows that Montag already doubts himself and his work. For Clarisse to state it out loud so succinctly shows fantastic character judgement on her part. Also, most people would not come right out and say something negative about the person that they are speaking with. They might think it, but wouldn't say it, or would find a way to avoid the topic. Clarisse flat out tells Montag that she thinks he is strange. That's honesty. I guess that's why people say "the truth hurts."
Along those same lines of speaking truth to Montag is this quote:
"You're not like the others. I've seen a few; I know. When I talk, you look at me. When I said something about the moon, you looked at the moon, last night. The others would never do that."
It lends evidence to her being honest, because it's not a flattering statement toward Montag. The reader knows Montag, and her statements line up with what the reader knows, so the quote gives more evidence to her being honest.
Clarisse is honest because she speaks directly about the world and her place in it.
The world in which Montag lives is predicated upon conformity. People blend in with one another. To speak with a sense of candor would cause individuals to be seen as different. Clarisse is honest because she does not care what others think about her. When she speaks about her place in society, she does so honestly: "I'm antisocial, they say. I don't mix. It's so strange. I'm very social indeed. It all depends on what you mean by social, doesn't it? Social to me means talking to you about things like this." The honesty she displays is disarming. Clarisse is honest in this quote because she is reflective about what it means to be social. She is honest because she is not fazed by what others think of her. She even dismisses these judgments as "strange." Montag is not accustomed to such a direct approach in talking about society.
Clarisse's capacity for reflective thinking is one of the reasons why she is so honest. This honesty is on display when she talks about how she "likes to watch people." Clarisse is honest about what appeals. She enjoys "people watching" and developing narratives about them. In this admission, Clarisse does not cave in to conformity. She does not say what others want to hear or what is expected of her. She is honest about what she likes to do and what she finds pleasing. In thinking openly and speaking about it directly, Clarisse showcases her honesty.