hmm, i have been playing a lot of tony hawk wasteland recently, and I honestly think they mark everything, because there are even places to grind in that game that are only textured on (one of them being in "the mall"). That being said, its probably definately possible to automatically recognize it (might be a little buggy though) by taking the normals of the sides of the thin object your grinding on and checking to see if they changed, if they have that means you need to rotate your players pan, and then check the flat part under them and if that changes, then you need to rotate the tilt.