In dozens of entries into China, no one has ever asked to see proof of a return ticket. Indeed, for many common entry scenarios, it would be impossible to show one. Crossing the land border with HK is perhaps the best example of this - what, should I show them my Octopus card to prove I can take the MTR back?
Based on my experience, I wouldn't worry about any problems from Chinese immigration. If the airline gives you trouble at departure, as others have suggested, offer to buy a refundable ticket on the spot and simply cancel it as soon as you land. I wouldn't worry about preparing this in advance, though, as I've never been asked by an airline either (this assumes a valid visa, of course).