Particularly with reference to hired tandems, or when you're first setting one up.
A bit of context: I hired a tandem today and went for a ride with my wife. I've been riding for about 20 years, she rides very occasionally (once every couple of years). She's 30cm shorter than me, so she goes on the back and I go on the front - which means it's a bit harder for her to diagnose what's going wrong with the bike as she's unfamiliar both with the components she's looking at, and knowing what 'correct' feels like).
The tandem we hired was a bit of a tank: an old hack that gets ridden up and down a path in a park a few times every week, the gear shifters didn't work (and the indicators were completely worn off the shifters), brakes didn't do much apart from suggest the bike might slow down at some point in the future, usual sort of decrepitude, but as we were only going on a very flat bit of tarmac, none of that was a big issue.
When we were pedalling, there often seemed to be changes in how much feedback and force my wife was getting/putting through the pedals - sometimes it would feel that she was pushing much too hard on the pedals, and at other times she felt that I was upping the cadence without warning - which I don't think I was. Could this be caused by the chain betwee the front and rear cranks being misadjusted - and if so, what can I do to check that before riding the bike away from the hire shop?