Actually, the easier way to tell is to just go to your application list and look to see if you have the SuperUser application. If you have it, then you can gain escalated privileges.. if it's missing, then you aren't yet rooted.

FAR simpler to just look for SuperUser in your app drawer because any app (including the su command) will depend on that app being available to provide those escalated privileges.

As for HOW to go about rooting, that depends on the device. Which device are you guys interested in rooting? I've rooted three devices personally, so far.