Efficient Marker Detection for Images
This is a saddle point. I would suggest using derivative operators to spot such discontinuities. In fact, Haralick presents a broad overview of these methods in his Topographic Primal Sketch.

The use of approximate Facet model would give you a speed boost. I used them for edge detection here.

There is also one implementation available here.



Even though this question and the topic is rather old, I would like to update that, the method presented in the following work contains no branching and is quite suitable for the shader:


  Birdal, Tolga, Ievgeniia Dobryden, and Slobodan Ilic. X-Tag: A
  Fiducial Tag for Flexible and Accurate Bundle Adjustment. 3D Vision (3DV), 2016 Fourth International Conference on. IEEE, 2016.