The tell-tale sign of nitrogen deficiency is yellowing which starts at the tip and moves back down the midrib of leaves. Eventually the leaf tissue turns brown as it dies and decays.
If you're trying to assess your nitrogen program and find leaves showing signs of deficiency now, it may mean the crop ran out of nitrogen late in the season.
How severe the deficiency is and how much it is likely to affect yield will hinge on how far the "firing" or N shortage occurs in the plant. If it's at the ear leaf now, that's not a good sign. Most corn has not black-layered yet, and is still trying to pump sugars into the kernel. If it is short of N this process won't be as efficient.
If firing is above the ear leaf then the plant has likely been short on N for some time. That means you can likely expect a sizable yield loss.
At this point in the season there is little you can do about rescuing this crop. You can, however, assess why the crop ran short on nitrogen. Did you not apply a high enough rate if N? Did you apply it last fall or early this spring before heavy rains? Did you not include a nitrification inhibitor, even if you applied most of it sidedress? This year areas which received heavy rain after sidedressing small corn may still be subject to N deficiency.
Remember that what you determine relates only to this year. It can help you plug an obvious hole in your system, but in another year when weather conditions are different, nitrogen may or may not prove deficient, even if you did exactly the same things related to nitrogen that you did this year.
Fields with firing well above the ear leaf are worth investigating as to what might have went wrong.