How to prove/evidence onward air ticket in the Online age ?

Hello, we are flying LHR-PVG-PEN-LHR staying in Shanghai for 5 days. I've checked all the guidance, and we definitely qualify for 144hr visa free travel.
But...how do we evidence the onward travel air ticket, when we can't check in (and therefore print) the Cathay Pacific flight out of SH until 48 hrs before departure, ie after we have arrived ?
Will the authorities accept a print of the booking reference or electronic information?