How do I know that the object has reached its goal?

I have an object which, when I press a key, should move to the target, how can I find out in the script that the object has reached its goal?

 

You may start a corooutine and regulary check if the moving object position is close engough to target. Or, you may create a event on that object and subscribe to it. Fire the event from the moving function when it ha approached the target.

 

have documentation? or what to look for?

 

Check the distance from the object to its goal against some thereshehold you define, say .1m, if its smaller, you're there.

 

Check this
https://docs.unity3d.com/ScriptReference/Vector3.Distance.html
https://unity3d.com/ru/learn/tutorials/topics/scripting/coroutines
https://docs.unity3d.com/ScriptReference/Events.UnityEvent.html

 

Don't ever use Vector3.Distance, its excatly the same as saying (firstVector - secondVector).magnitude with an extra func call, furthermore, use the sqaure magnitude and compare against a squared version of your distance, it costs you 1 multiplication(squaring the distance) but saves you a root function, a net gain.