Smartcards & Train Conductors

Can anyone who uses a smart card advise - my commute is two trains, and I'm looking at using a smart card. I'm guessing I just beep in when I start my journey and beep out when I get to my last destination. However on my route there are often a lot of train conductors as well as the machines. How do conductors/ticket barrier workers do with smart cards? How can they 'check' them?