To be sure I choose a reliable SEO, what questions could I ask to feel more confident it is the right one?
If I ask "how or what" then obviously they keep it vague (which i understand, nobody wants to actually TELL the techniques).
The most obvious of course is if they don't rank well on anything…not a good sign. don't buy excuses that their sales guys will feed you about being too busy or working with too many clients.
do some research yourself. join some forums and read blogs. ask questions. there is a lot of information available for you free on the net that can help you make your decision.