You need to cast a minimum of three rays, I think there are some edge cases that would require more.
You need to cast three rays with the same direction, in a triangle - let's call them rays A, B and C.
The angle ABC should be at 90 degrees along the direction of the ray.
Assuming all three rays hit, you should calculate the vectors B -> A and B -> C
Normalize those vectors.
Cross product those two vectors.
You should have the surface's normal.
You'll have problems if one vector doesn't hit (the very edge of an object) and also at the intersection of two objects, but if you make the distance between the rays as small as possible, you shouldn't really notice this.
It also won't work at very acute angles.
Or just wait for 1.1