Most publications list either 15 or 16 inches of width from tip to tip of an alert whitetail deer. Generally, per biologists in our area, a deer measures 14 inches from ear tip to ear tip. A best practice is that if one can see an inch outside the ear on either side one may conclude it is a 16 inch spread. As far as beam lengths go, there are way more variations on what to look for, but generally if the beam extends to the nose it will have 22+ inch beams.
Recommended best practice for those that are trying to field judge deer is to wait until you see that that there is NO question about whether it will make the specs or not. If you have to wonder and try to decide and waffle back and forth, then generally that deer is going to be too small.