Yes, you can check if she's egg bound, but if not I wouldn't wait for a test with cocci: speedy treatment is important. Since you have the corid, treat her. It has been wet and warm here, so cocci is definitely a possibility. If she doesn't respond well within a couple of days it's time to...
We need more details to help. Any changes lately (feed, weather), how are tou keeping them, where did tou get them from, how old is the hen, what if anything have you tried?
Cocci is a possibility - you can get Corid at the feed store to treat her - search this site for correct dosage (give her...