How would an airfoil react if it was flown backwards?
What would happen? Flow separation on the suction side, but it would still produce lift like a regular airfoil. The L/D ratio would be lousy, however.

Only at a small angle of attack range will the wing show attached flow on both sides: When the stagnation point is right at the tip of the trailing edge. This behaviour is similar to that of a flat plate and produces a rather limited range of useable lift coefficients and substantially more drag than when used properly. But still you get more suction on one and more pressure on the other side. The blunt rear edge will cause flow separation at all angles of attack and substantially increase pressure (or form) drag.

An airfoil with a blunt trailing edge has an advantage over a flat plate because it will perform acceptably over a slightly larger angle of attack range, but still this will be no comparison to the behaviour with the blunt side facing forward. Indeed, the front of an airfoil needs to be blunt to allow its use over a larger angle of attack range, while its rear end needs to be pointed to reduce the area over which the flow separates.

Now I feel I should drop a line or two about the cause of lift. Essentially, a wing creates lift by accelerating the air that flows around it downwards. The inclination of the airfoil will already be enough to cause this acceleration, regardless which side faces forward. The plot below shows how several airfoils perform over the first 180° angle of attack. 



Lift coefficient over the first 180° angle of attack (picture source)