if i'm planning on hatching eggs i make sure to have an incubator on hand. you need to be sure that a hen is broody before you buy the eggs or else they could decide after 2-3 days to not sit on the eggs and you run the risk of the eggs no longer being viable for hatching. it is very hard to make a hen just go broody without the signs already there.
if you had started this a week before you planned on ordering eggs i would suggest keeping some fake eggs, golf balls or some rocks that may look like the size/shape of an egg to test her willingness to stay broody. if a couple days passes by and she is still allowing you to get close but picking at hands and screeching at you she is probably going to be good with it. if you attempt to move her from the nest/box and she instantly heads back she is willing! switch out the fake eggs when they arrive and watch her to be sure she doesn't just give it up or eat the eggs. i would also recommend giving her a secluded area to keep the nest free of the other birds. the others will tend to roll the eggs too much or crush the eggs while trying to lay their own.
so in Kevin Costner's 'Field of Dreams' saying: if they are broody they will hatch it!