if you have multiple kegs and a kegerator and sometimes take your beer to parties or homebrew club meetings, pills
this becomes a more serious issue. Different people have different processes, but this one is mine.
For simple identification, I label each Corny keg on the front with its contents, as well as my name. The contents label makes it easy to identify, obviously, and the name label is handy for when I go to my homebrewing club, where we have at present four guys who keg their beer. It just ensures that I go home with the same equipment I arrived with.
These two labels go on the front. I defined the front as being the side facing you when the input side is on the left and the output on the right. I read once that the proper way to install the lid of the keg is to have the lever pointing toward you when the carbon dioxide hookup is on the left, so in my mind I decided that was the front of the keg. It was arbitrary, I admit, but it established a standard for how to do this.
I also make some markings on the gas side of the keg. One piece of tape shows the size of the fitting. Ball lock Cornelius kegs typically have one of two fittings, either 11/16" or 22mm. Being able to see this readily allows me to grab the right tools the first time without having to mess around too much trying to figure it out. This is where the pin lock kegs have an advantage: one tool to take any of them apart. Granted, it is a specialty tool, but there is only one size.
The other marking on the side of the keg tells me the date I last rebuilt or serviced the keg. I have been on a kick to replace poppets in all my kegs because I have had some recent failures, so I label when I replace both the seals and the poppets. If I were really organized I would keep a spreadsheet of this information to establish a proper maintenance schedule, but I don't guess it's really that important. I only have seven kegs, one of which I have not rebuilt or even used yet. This is not a big operation I am running here.
Finally, I try to keep my kegs as clean as possible, and often have a keg or two cleaned, sanitized, and sitting in the closet. To designate which homebrew kegs are ready for use, I put a piece of tape with a check mark on top to denote that it is ready for use. Again, this is just a quick visual cue that tells me whether a piece of equipment is in good order. I can just grab it and go.
The materials I use for this labeling are pretty simple. A Sharpie pen and a roll of the cheapest cloth medical tape I can find do the trick nicely. The cloth tape is pretty rugged and holds on well even through washing the outside of the keg. It also peels off easily, should I want to change the labels at any point.
Whatever method you use, you might find it useful to start labeling your kegs and establishing this set of good habits so you know whether your equipment is ready to use or not. Do you have a method for this? Let us know.