How do you know if the campsite is full. I dont wan to drive all the way and not be able to stay.

Walk ups only. It's a rough bumpy ride, if you don't show up early, just go somewhere else.

So a Thursday around 5pm I probably wont get a spot? Does this include the more primitive sites as well?

Then I would choose another campground. There is really no way to know until you get there.

It fills up every day, so I would recommend somewhere that you can call ahead and reserve.