If the value check box is automatically controlled, it would be considered a calculated value, so, although you do not mention storing it in a field of a table, I want to stress that you SHOULD NOT store the result in a field of a table....(ie: Do not store calculated values!)
So ... with that ... to accomplish your task, which to me seems ... well ... redundant because the user can see whether or not a value is placed in the text box or not!
... First you would create a check box on the form, then set the Control Source property of the check box to something like this:
=(Len([NameOfTextBoxThatControlsTheCheckBox] & "") > 0)
What this will is check the length of the value in the check box, if its more than zero, the express will be true, thus "Checking" the box, otherwise, the box will be UN-checked.