Everything will be okay! She has a nest box- so if she _is_ gravid, she has a place to lay. If she continues to eat with a healthy appetite, I would just go ahead and feed her. If you feed her every 10 days, you can see if she is gaining weight, or losing weight. Over the course of say a month. Three or four feedings.
Have you looked around for a vet who sees reptiles? I don't think it's anything urgent, but you could take her (and one of her fresh poops) in and just get a physical for her. See if she has parasites. I wouldn't be alarmed if she does- every snake I have ever checked has had them. They are easily treated. The vet ought to be able to tell you if she is gravid. He ought to be able to feel any eggs.
But- you're prepared for her to lay if she is- so it isn't an emergency to find out if she is or not. If she is, she will go into her lay box one day and stay there until she lays. She may go in and out a few times, over a few days, until she settles in. She may _like_ the novelty of a lay box and spend time in it even if she isn't gravid. or she may show no interest. The only thing you have to worry about, a little bit, is do you want to incubate the eggs? If not, put them in the freezer for 24 hours and then dispose of them. You would have to think about are you prepared to care for an house all those hatchlings until you can sell them. If you do decide to incubate the eggs, all you really need is a spot in your house that stays around 80F. You especially don't want heat spikes up into the 90's. You can buy a simple incubator from ThinkGeek for $100.
Okay, so if she isn't gravid, just overweight- or more accurately, a little overweight and a lot out of shape, you can work on that. She probably came from a home where they did nothing but feed her. Now you have her and enjoy handling her, so exercise won't be a problem for you (and her). A healthy, big female cornsnake typically weighs between 500 and 700 grams- so she's in the right weight range, but she has those fat deposits. It would be nice to see if you could tone her up a little. And definitely not add to it!! I have not _heard of_ a snake dying from being fat, so don't get all worried, but it would be healthier for her to increase her muscle tone.