Bakery

Question

I have heard that if you confirm that a bakery or a bagel store only uses kosher ingredients that you can eat there. Is this correct? If so, where do we draw the line, that is, when do we need a mashgiah and when not? I assume that as the food production becomes more complicated in a store (e.g., chocolate covered doughnuts etc.) you need a mashgiach, but where do you draw the line?