Can I use Wukong's passive to detect the presence of enemies?

Wukong's passive is as follows:

(Innate) Wukong's armor and magic resistance are increased by 4 / 6 / 8 for each nearby enemy champion.

Using this passive, are you able to sense the presence of enemy gankers or junglers so long as they are within range, even if they are in bush? What if they're stealthed? Or does the bonus armor and magic resistance only activate if you can see the enemy champion?

The answer is: No, Wukong only gains these bonuses for visible champions within range, so stealthed champions or champions hiding inside a bush in which Wukong has no vision do not activate his passive.

Still, you could theoretically get some useful meta-information from this ability! In case you were facing an enemy Wukong :-P Wukong is a staple solotop laner, and as such he has to be very careful about the possibility of suffering a gank from the enemy jungler (which in this hypothetical situation would be your team's jungler). As a consequence, solotops tend to place a ward in a couple of critical bushes in the river in order to get some map awareness.

If you suspect that the enemy Wukong has placed a ward in one of these key bushes, you could enter them while Wukong stays relatively close (you do not want to give him an easy kill, and after all the radius of activation from his passive ability is 1400, which is quite large) and notify whether or not the status icon associated to his passive disappears (you can see it if you move the cursor over the enemy champion). If it does not disappear, that means that the enemy Wukong retains vision inside those bushes... Which means that they are probably warded.

And then you could warn your jungler not to waste his precious time waiting to gank him :-)