I have a 2006 XJ8 and want to replace the rear directional signal light with LED's. How do I know if I need canbus or regular LED bulbs?

Take an indicator bulb out and if you get a message on the display telling you a bulb isn't working, then they are indeed canbus controlled.
I'm pretty sure they will be.
You can actually still use non canbus bulbs, but you will need to install inline resistors to prevent blown bulb messages. I converted my Volvo in this way and it was pretty straight forward.