Result is undefined if computer isn't on, so any value is OK, no tests necessary.
GP said for the 'non-true state', not the 'computer off state'. I.e. verify that the computer is off, given that the fn returns false (which is valid only as part of its undefined computer-off response).
It's actually pretty straightforward:
turn_computer_off();
Assert(!is_computer_on(), "Computer should have been off but it's on.");
The opposite is a bit tricky, though.
This assert doesn't work like you think it does. If the computer is off is_computer_on is undefined, so if this is called when the computer is already off it could return a nonzero value, thus resolving to true even though the computer is already off.