Entity looks at other invis Entity

I need desperate help for a mob I´m creating. It´s basically supposed to look at nearby invisible entities, prioritizing the closest one, but I can´t find a way to implement that. 
Does anybody know a procedure that could make that happen?

If there are other ways of making the invis entity(or entities) visible (by making them glow in a radius for example) please tell me! I just need to find a way that the puppet can find nearby invisible entities. Please help!