I'm not female but I think I'd get offended if my gf checked other people out in front of me. Similarly, I'd expect her to get offended if I did the same.
The way I see it, you're with someone, then the other people around don't matter one bit. They can be sex on legs for all I care, if I'm in a committed relationship then they aren't worth a glance. If I was interested in checking out random people and drooling over them then the best place for it would be outside a relationship.
My partner doing this would make me constantly wonder at whether they were really all that dedicated. If you're out with someone, and can't be fucked looking at them because you're too busy eyeing off random strangers, to me that spells a lack of respect. Even if my partner did spot someone who took their fancy, I'd at least expect them to have the self-control not to stare conspicuously stare at them while with me.
Unless of course you're trying to convince your partner that it's only a temporary relationship, and that you'll bail as soon as someone better looking comes along. I don't think I'd see the funny side of it as a 'joke' either.