The approach I would use also works in the related situation where you're trying to help someone fix a problem and you want to be sure they've tried all the obv

The approach I would use also works in the related situation where you're trying to help someone fix a problem and you want to be sure they've tried all the obvious things. "I'm going to sound like a tw@t* saying this, so I apologise in advance, but I have to ask anyway. X" where X = "Did you try ..." or X = "Why didn't you use ..." * change the expletive to whatever's appropriate given whom you're addressing.