kill 0 basically asks that PID if it's still alive or not. If you try a kill 0 on a PID that doesn't exist, you'll get false back. Be careful, though. Because Unix PIDs are recycled, there's no guarantee that the process that currently has the PID that you're looking at is the process that you think it is...
thor
Feel the white light, the light within
Be your own disciple, fan the sparks of will
For all of us waiting, your kingdom will come