They should offer etched knobs with DB or 3x on them so people can remember.
Here is the easiest way to tell:
1. Plug your bass into an amp and put the pickup selector into position #3.
2. Very lightly tap each of the 3 pickups, ideally with something made of non-stainless steel. Dingwall was kind enough to include a set of wrenches with your bass that would be perfect for this task. (Place a piece of paper on the pickup first to protect it from scratches if you are using a screwdriver or other pointed object.)
3a. If only the middle pickup responds to the light tapping, you have a D-Bird Switch.
3b. If only the two outer pickups respond to the light tapping, you have a 3X Switch.
If you can't tell the difference, either (a) you are tapping too hard, or (b) you have been playing too much without hearing protection.

In this case, remove the rear cavity cover and examine the circuit board attached to the pickup selector. It will be printed with either "3X" or "DB".
Some folks might say that another way to tell is by placing the pickup selector in position #4, plugging in to a big amp, and turning it about half way up, then start playing. If you are knocked over by a massive wall of sound unlike anything you have ever heard before, then you have a D-Bird switch.