If there is more than one way, please list them. I only know of one, but I'm wondering if there is a cleaner, in-Ruby way.
If it's a process you expect to "own" (e.g. you're using this to validate a pid for a process you control), you can just send sig 0 to it.
>> Process.kill 0, 370 => 1 >> Process.kill 0, 2 Errno::ESRCH: No such process from (irb):5:in `kill' from (irb):5 >>