Microsoft’s Raymond Chen tells the story of a customer who complains that the keyboard isn’t working (blogs.msdn.com/oldnewthing/archive/2004/03/03/83244.aspx). Of course, it’s unplugged.
If you try asking them if it’s plugged in, “they will get all insulted and say indignantly, ‘Of course it is! Do I look like an idiot?’ without actually checking.”
“Instead,” Chen suggests, “say ‘OK, sometimes the connection gets a little dusty and the connection gets weak. Could you unplug the connector, blow into it to get the dust out, then plug it back in?’
“They will then crawl under the desk, find that they forgot to plug it in (or plugged it into the wrong port), blow out the dust, plug it in, and reply, ‘Um, yeah, that fixed it, thanks.’”
Many requests for a customer to check something can be phrased this way. Instead of telling them to check a setting, tell them to change the setting and then change it back “just to make sure that the software writes out its settings.”