Hinterfragen
To question what lies behind something. Duden: to ask about the background, conditions and foundations of a thing. The usual phrase is etwas kritisch hinterfragen.
Jemandem auf den Zahn fühlen
To feel someone’s tooth. To examine a person very critically.
Nachhaken, nachbohren
To hook in again, to drill again. To keep asking until the answer is complete.
Wie kommen Sie auf die Zahl?
How did you arrive at that number?
Hand und Fuß haben
To have hand and foot. It is said of a plan or an argument that is well thought through. Der Plan muss aber Hand und Fuß haben.