For one, if no is going to mean no, then I want to hear it. Why should we let them off the hook with body language.
It's sound of you to read body language, it's a good tell. But you can't get more obvious than "no thank you." From her lips to my ears.
Beyond that, it's not simply an ask-out. You have to strike up a conversation and use some technique. Body language and facial cues can shift a lot. If it's still not going well, you can end it with or without asking for a number. But what have you got to lose?
I had a first date a couple of weeks ago, it went fine, no fireworks. No encouraging body language beyond her being simply nice. I went for the kiss anyway. Got the cheek. I laughed and said "well I had to try!" to her. BFD.
Let me ask you this, you ever have a girl that you have great chemistry with but she still turns you down?