I've never been asked by passport control to show a return ticket. But I have often been asked how long my stay would be and I'm not sure how you would answer that. You would probably raise suspicion with whatever you said.

However I would definitely expect the airline to pick it up. I was on an award booking that somehow showed up as a one-way (not sure why, it wasn't) and I saw a flashing bright red line on the doc check agent's screen saying ONE-WAY. As a Canadian going to the US and Canada that wasn't an issue but it still caused the agent a noticeable pause to double check. I would definitely expect most airlines to question it.