I would honestly hope you guys wouldn't back off if she showed no inter est the first date. Sometimes its nervousness, or akwardness, hell she might be having a bad hair day and be self concious. Those are only a few reasons I can tell you a girl would not seem 100% into YOU, on your first date.
This IS great advice coming from the very gender in question BUT ma'am here's a few things that occur to me....................
IF a girl had an acceptable time and would be open to the possibility she should maybe send an email or call him with some small apprecation for spending time with her. This would let him know she MIGHT have had a bad day and that she's open to him asking again if he's still interested.
Savanna is right in that we all sometimes suffer from nerves, shyness, or feeling awkward in person initially and the phone or email lets us speak without that kind of gentle pressure. Women also need to know if they act uninterested and do nothing to say otherwise most guys will just not want to waste more time with someone who doesn't give more clear signals.