How do you know if you've scared a girl off?

If shes outright with her feelings she'll let you know but if shes shyer than that then she'll probably avoid talking, seeing and any other forms of communication. To put it another way: The two possibilites are 1): You did scare her off. Bummer. Or, 2): She is very shy. In which case give it some time (a couple of weeks) and gently try again. Then you'll know for sure.