I've always found it easier to just ask the question within the question. To either guard.

"If I were to ask you, 'which path is safe?', what would you say?"

If you asked the truth teller he will Indicate the correct path because he would have told you the correct path anyway. If you ask the liar he originally would have indicated the bad path- but now he has to lie about what he would have original told you and will now indicate the safe path. Asking what the other guard would have said just kind of adds another unnecessary logic wrinkle in my mind.