This is an excellent idea and one that professors often ask about so it is on development wish list. In the meantime, the easiest way to keep track would be to print out the list of questions on your assignment.
On the Assignment Manager page, each assignment has a drop down menu called actions and one of the choices is List Questions. Use this to see a list of questions on your assignment, print out this page and then use it as a reference when creating your test.
I hope this helps.