if the quadratic is a u shape or what a call a smilie face then it is a minimum if it is a frown or a .. kinda n shape or what i call a frowning face it is maximum normaly the quadratic has a negitive sign in front of the x^2 to be a frowning face~ ^------^
also u can tell as in a u you can't go below the lowest point of u and the same with n... u can go above the highest point in n when the graph is drawn....