It can be very intimidating to just go for a kiss if you are not sure if the advance is wanted. Good way to test the water is to get physically close to her by - for example - watching some movie and sitting on a couch or laying in bed together. Maybe ever so slightly touch her leg with your leg and see how it feels for you and if she breaks the contact or keeps her legs where they are or even gets closer to you on her own. In my limited experience this can give you a pretty good idea if she is physically into you or not.