When taking the MTR in Hong Kong, one has to swipe one's Octopus card on the way in and out. How can I know ahead of time whether I have to swipe my Octopus card when exiting a bus in Hong Kong?
There are some bus routes on which have two-way section fares, allowing you to qualify for a lower fare by swiping your card whilst getting off the bus:
(2) and (3) At present, there are 38 bus routes with Octopus two-way section fares. Details of these routes are set out in the Annex. In 2017, the average daily passenger trips made on buses under the Scheme was 544 000. In 2017-18 revised estimate, the Government's reimbursement of the revenue forgone to the operators under the Scheme is $1.1 billion. Currently, bus companies do not have information regarding the data of passenger trips enjoying two-way section fares.
The complete list of these bus routes can be found in this PDF file (updated in 2018).