Part of the key to a successful ticket check in my experience is the judgment call about when to stop checking.
It's all very well checking tickets up to the last second before a station, but a station call not only has to take precedence for obvious and well-worn safety and punctuality reasons, but it also offers up a multitude of distractions whilst dealing with an incorrect ticket / ticket sale / fare dodger etc. The potential for somebody to walk away and sit somewhere totally different, or start tearing their receipt off the PIN pad themselves and jam it up, or decide not to pay an excess, means that there is a point at which you have to really stop checking because you won't reasonably have time to do much about any validity problems or complete a sale before the next stop.
There are exceptions to this rule. I work on some routes where it is so commonplace to renew/buy tickets onboard that passengers can easily be trusted with a Thyron PIN pad. There are also times where checking as you pull into a station is OK because you don't need to make any additional announcements, the station is staffed so any fare dodgers can be escorted away, or something like that. But often, especially on lines where you are effectively the only member of customer-facing staff, a ticket check can throw up a whole lot of things you need to deal with but can't when you are arriving at a station and need to release the doors and/or dispatch.