It's been a few years since I've used it, and that was only a couple of times between going between a couple of stations, but that's not how I remember it. Some stations had ticket scanners at the entrance to the platform but some didn't.

I remember the Palo Alto station very clearly because I got on once thinking I had grabbed my ticket from the vending machine when I had actually grabbed the previous person's receipt because I was in a hurry. I got on the train and once I realized I was worried about the fare inspectors coming on and catching me, so that station definitely does not require a ticket to get on the platform. Maybe that's just classism with respect to Palo Alto?

Edit: never mind, it was Caltrain not BART.

But CalTrain does have employees occasionally walk through the cars scanning Clipper cards.