I have never been asked to proof or return/onward trip in any country. Not even in China back in the days where they said you have to show a valid ticket at immigration at a big sign next to the immigration officer. It was always delegated to the airline's check-in agent/system. In general, we can all agree it is a pretty BS requirement anyway, as no one can force you on that return/onward trip. I think the basic idea some point in the past was to correlate the passenger manifests with the immigration records to flag people who do not leave the country as they indicated at the time they entered it, but this as also dropped due to the whole impracticalities around it. And it is much easer to just flag the passport number after they overstayed their visa. And even that s not done in most countries but they only find out in a routine police or border patrol check.

Anyway, for OP... I would assume to get denied check-in if I cannot prove return/onward travel. Easy solution, have the next flight already (refundable/flexible) booked.