How do I know when the Fnet in y direction is 0, any hint i have a test tomorrow?

Look at the in the y-direction

The easiest way to say if Fnet in the y direction is 0 is to ask, "Is there any acceleration in the y-direction?" For most problems, you can intuitively know this. If the problem does not allow the object to accelerate in the y-direction, then the net force in the y direction must be zero.

Examples where the acceleration in the y direction is not zero: freely falling object, object moving down ramps, objects pouncing up and down, pendulums swinging back and forth.

Examples where the acceleration in the y direction is zero: any stationary object, objects moving across a horizontal plains, object falling at terminal velocity, objects moving around a slanted race track where the centripetal force prevents the car from moving sliding on the slanted track.

In the later examples, Fnet in the y-direction is zero.