the bucal cavity will expand or she may only have a few eggs which won't make her mouth swell much at all. In a few days as the eggs grow into fry the eggs will darken and so will her "chin" which will confirm it without actually looking in her mouth.
i disagree with this advice, it is easy to do and far better for the malawians as they don't eat while holding. If you leave her to spit she will loose a lot of condition and you run the risk of loosing her.
I often check if a fish is holding, just net her and open her mouth with your finger nail or something pointy but blunt, I use a roofing nail which works really well. I strip around day 16 or 17, that way they are free swimming and the female doesn't loose too much condition.
here's a good video showing how easy it is. just stay calm when you first do it, i have done it dozens of times with a wide variety of africans and it is really easy once you have done it once or twice.
http://www.cichlid-forum.com/videos/stripping.php