move_and_slide have a couple of parameters, the 2nd one is the floor normal, if you set it, like (0,1,0) [pointing up on Y], then when checking it should say true if is the close to the normal (including slope tolerance).
The last parameter is important too if you want to allow some specific angles to be taken as "floor" (like on slopes or irregular floor), by default is around 45º.
Look here or the internal help for a bit of information that may give you a hint on the full usage (still no detailed description, this method may change again before 3.0):
ps: I have seen discussions about
move too so be prepared for more changes.