Suppose you go on a date with this girl.
During the evening, your realize that she is a nice person, but you would not go further with her. What to do?
Something which really works is first to be friendly in the beginning of the date. If you don't feel the chemistry building up, you simply don't move to a more intimate level. You keep it friendly.
It's important that the two of you have a good time, whether you date further or not. That's what keeps it open and makes it worth no matter what.
You don't go on a date to check her out. You go on a date to have a good time.
Focus on fun and quality time. No sentimentalism. No serious talk. What she will remember is a good evening with a nice guy.
If you don't feel the chemistry, keep it friendly, respectful, give her a couple of compliments but simply don't respond to the invitation of being intimate. No need for the awkward moment where you break her heart... No.
You can say something like: "You know, I had a great time this evening. You are really fun to be with. We could do that again sometime."
Now, if you say that, but don't make a move to be closer to her, or keep it vague, she'll think: "Well, he does not seem to be interested by more, but true... we had a great evening. He's a nice guy"