I'm new here (hi Andrae) but the current plan is that you will be able to set up Presto on GO to have a "regular route" (e.g. Ajax to Union or whatever) that you regularly take. When you tap in, it will deduct for your regular destination. If that is where you are going, you will not need to tap out. If you are going somewhere else, you will need to tap out at that station and Presto will make the adjustments. And yes, there will be roving inspectors with portable readers.
Not sure how they would handle the scenario of confirming that someone who normally rides Ajax to Union but is staying on to Port Credit will actually tap out when they get to Port Credit.
Having said that, there are a lot of discussions going on amongst the Presto team and various transit providers, and it's possible things may change.