After being in the cattle business all my life and being able to identify 200-300 different cattle and know each's ID number (and usually sire & dam) by memory, I look at differences in coloration in deer first, then physical differences.(Shape of head, length of ears, fullness of brisket, etc.)

Every deer is marked differently. All you have to do is recognize it.

As Troy said. It takes a lot of observing and really studying the different characteristics.

It's really easier to differentiate deer than it is in a pasture full of solid black, solid white, or hereford cattle.

It's simply something you learn to do and not think much about it.

Look hard at each deer you have on your wall. They are all different but in a subtle way.