So, while flushed lips can indicate arousal (and make your lips more noticeable)the fact that your lips weren't noticeably different doesn't mean you weren't attracted to the person you were kissing. After all, the best indicator of whether you want to make out with someone is if you feel a desire to do so, not how your lips react to them.
As for whether or not people can tell, if your lips do swell or redden a little during kissing, it's not going to be such an extreme change that it's obvious from yards away. Someone close by you may notice, but even then they'd have to be looking for it. Does that make sense?