I don't know how many times I've asked my kids to clean their rooms, and had them come back repeatedly to ask me to check if it was clean enough to excuse them. Is it clean? "No, put the clothes in the dirty clothes basket." Is it clean? "No put your books away." And on and on... That's when you want to give up and clean it yourself!
I read something that suggested to give clear and concise guidelines for what you expect. What better way to do that then with a photo?
I took a photo of the kid's rooms clean, then printed and laminated it(just so it wouldn't end up in a crumpled pile of garbage, but it's not necessary). Now, hopefully, they will have a guideline for what the room looks like clean. Make it look like the picture!