Open questions is the answer... don't ask them a question they can just say "yes" to.
"Press the 'info' button on your TV. What station does it say you're on?"
"What have you done since you discovered the problem?"
"Tell me which lights are on the router."
"Type your password into 'notepad'. Does it appear correctly?" (nobody wants to consider if they left their caps or num lock on)
"The call is coming from inside the house!"![]()
That technique will also help you spot keyboard problems. I remember one time I was repeatedly getting password errors. When I finally got on, I tried typing the whole login sequence in notepad and discovered that the "tab" key, which I used to move from user ID to password, was intermittently producing "<tab>\". Time for a new keyboard.