Can you detect when a wraith cloaks? It's just a random question.

Not pracitcally, no.

Of course you can, It cant be shot by other units without a detecter anymore.

How would that be detected? If the unit chases the wraith?

Of course, if a unit can see a wraith, itll follow it if it shoots once.

Place a scourge owned by an enemy player over it. Wait at least 500-1000ms and check if the scourge still lives.

The scourge should be invincible and have 0 dmg (=0,5 dmg).

When you want to maintain the wraith's hps and/or need scourges with damage elsewhere you will need a grid.
place the scourge a bit away from the wraith, center a 1x1 pixel location on the scourge and again after at least 500-1000ms check if the scourge left the 1x1 location.

After you checked remove the scourge.

When you need an invisible method you can mess around with arbiters near, but out of view, or a cloaked wraith as detection unit (need greater distance)

Or if you disable enable enable the scourge, they cloak completely.

Wouldn't that crash the game then? I thought that all Zerg units would crash the game

No, it's more like Protoss units, Zerg units take that much better than most, just make sure the player can't even see them while their being disable enable enabled.