Always Ask Why? Demand proof!
It really frustrates me when I get emails promising that I'll be a 'guitar master in a day' if I'm happy to part with lots of money. It's all bs, if it doesn't take work, it ain't going to make you better!
So ask why all the time! If you teacher tells you that you should do something a certain way, ask why?
I try and make the whys clear in my lessons but I'm sure there are holes and so I encourage all my students to push the why, it'll make me a better teacher, and them a better musician!!
You should also ask yourself why too! It'll help you find answers to questions you didn't know you had ;)