How to determine which control on form has focus?

I've read elsewhere on here that to capture "Enter" key stroke in a text box and use it as if pushing a button I should set the KeyPreview property of the form to true and check the value of KeyDown.

I want to be able to use this functionality on several TextBox controls which each are associated with a different Button.

My question is how do I know which control caused the KeyPress event? The sender is listed as the form itself.

G