The easiest way to find out is to go to your Controls and look.
Also, there is a small "+" sign under the binding. If you click on it, there is a menu item that will let you choose whether you want your cargo hatch on "hold" (you have to hold the button to keep the hatch open and when you release it, it closes) or "toggle" when you press the button the cargo hatch opens and when you press it again, it closes.
I strongly suggest using the "toggle" option, I bet you have it set on "hold" so that even as you're pressing the correct button, the hatch doesn't stay open.