Until now, the only way to tell whether a test still had answers waiting was to open it and start grading. There's now a marker for it right in your list of tests.
The Grade test button shows a small green dot whenever answers are still waiting on you — hover over it and it tells you exactly how many, like "3 answers need grading". It keeps up as you work, so once everything's marked the dot goes away.
Only answers that actually need you are counted: questions that grade themselves, and anything you've excluded from scoring, don't add to the total.