How can I determine if a object is being show on screen?

I need to know what is currently seen by the active camera or what not.

Tried onBecameVisible, but not really working

You can check if a particular object is in view with GeometryUtility.TestPlanesAABB, or even just with Renderer.isVisible. Note that "in view" vs. "is visible" may be different and not apparent (e.g. if an object is not on screen, but its shadow is, it is technically visible since it needs to be rendered).