Ticket checks are all done onboard HEX trains and the train staff did not have any readers so all you had to do was flash your Oystercard. I did not get anything deducted on both trips I made. In fact, there were some pax who wanted to buy HEX tickets onboard and the train staff asked if they had Oystercards first.

I made sure to ask at the HEX ticket office if my Oystercard was valid before travelling. You can also ask the on-board train staff to confirm before boarding.