I tried writing on them with sharpie, but the markings fade over time. Those rubber key-head enclosures were good at first, but they wear out, get scroungy and are hard to take off the key ring. I soon learned to identify the keys by the way each one was cut. But usually, I just guessed until I got the right one. But hey, I was right 33% of the time!
A simple solution occurred to me: drill holes in the keys to identify them. One hole for front door, two for the apartment door and three for the one on the deck.
This is an earth-shatteringly simple Instructable, but there are a couple tips worth reading.
Step 1: Get Out Your Household Drill
Step 2: Clamp the Key Down and Drill the Holes
Drill the holes for each key.