64-bit: How do I find out whether I can run a 64-bit Linux OS?

I am surprised how often I get into this discussion:

Someone: How do I determine whether I can run a 64-bit build of my favorite Linux distribution?
Me: Only if you have a 64-bit processor.
Someone: Great, but how do I determine that I have a 64-bit processor?
Me: Well, if you are already running a Linux distribution, you just need to run this:

grep 'flags.*lm' /proc/cpuinfo

If you get any output, then, my friend, you are ready to jump into the 64-bit world.

Update: Just because you are running a 64-bit OS doesn't mean you have to. You are free to run a 32-bit OS, if you wish.