Shy/nervous is one thing, but someone who wants to go forward with things will at least try to reciprocate, even if she's shy or awkward about it. If a girl is withdrawing from an encounter by either not reciprocating at all, trying to make small moves to get away, or mentally checking out from the situation, she is likely uncomfortable. Pressing on in such a situation is not cool at all.
If you're not sure if she's comfortable, ask if it's okay to keep going. If she does not answer or if she doesn't seem into it, take that as a no.
The point is, you're not supposed to be looking for resistance. You're supposed to be looking for a desire to continue. No response is not consent. You're not supposed to think "She didn't specifically say no, so therefore I can keep going". You should be going "Is she acting like she wants to keep going?" If the answer is anything besides yes, then stop.