How do you know if a praying mantis is pregnant?

I have one in captivity that was really thin at first and ate everything in sight but a week later it is fat and has stopped eating except for an occasional moth..it hangs in the corner of the habitat and doesn't try to catch the bugs we put in. Could it be a pregnant female? or is it more likely just an overfed mantid?
Answers:
if it makes an eggcase it is.
http://www.ivyhall.district96.k12.il.us/.

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over fed.

they don't get pregnant they lay eggs and without a male you won't see those.

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It's fat and lazy with clogged arteries

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ig the daddy mantis is laying very still at the bottom of the habitat and he has no head.

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A praying mantis doesn't get 'pregnant', she stores the sperm from the male in a special area just inside her ovipositor, and the eggs get fertilised on their way out..!

As your mantis is fat, there's a good chance that it's a female! Males are not so fat.! As the mantis was very thin when you got it, it had probably only recently moulted (or maybe it just couldn't find any food.)

Look at this pic - at the underside of the female mantis.
http://www.bugzuk.com/hierodula%20parvic.
I know it's not very clear, but you can count 6 'sections' on the underside of her abdomen. Males have 8 normally. It's not always easy to check, but if you can see those sections, count them - it's a good way to tell the gender! Plus females get much fatter than males!

Now to YOUR mantis -
Question!! Does it have wings?

a. If the answer's no, then your mantis may have been stuffing itself in preparation for moulting. It will need a space to moult with a height that's at least 3 times it's length. They like to hang from a twig, or from some sort of netting material - I often use those netting bags that you're supposed to wash clothes in! They normally stop eating a day or two (or three) before a moult.
WARNING! Don't leave crickets in your mantid's habitat if it might moult! They are very vulnerable after moulting, and crickets DO eat their wings, wing covers, and legs!

b. If the answer's yes, then your mantis may well be thinking about laying her ootheca / egg sack, but if she hasn't mated it will probably be infertile - although a few species allegedly can produce a small number of mantis nymphs from an unfertilised egg, but I've never seen it, and I don't know anyone who HAS seen it..!

However, as you've only had it for a week, you won't KNOW whether she's(?) mated or not, but if she has wings, it's a good possibility! Store her ootheca well, preferably outside where nature intended, or put it in the fridge for a couple of weeks in mid-Spring. They need a winter usually, although tropical species are mating and hatching all year round.

Of course, they DO stop eating sometimes just because they're full up . but not often..!! There's allegedly a risk of them eating so much they burst - but again, no-one I know has ever had this happen to their mantis!