I think another big difference between a good jg and a bad jg is that a good jg knows where the other jg is or where hes gonna be. Like its not that hard to know what lanes are easy ganks for their team. So its easier to be close to defend. Sometimes counter ganks are better than a normal gank.
So even though you cant always know where the enemy jg is with wards (they help tho) you can know what moves the enemy jg wants to make. Like lanes to gank or objectives to take (scuttle crabs, drags, herald, towers, barons, buffs, whatever). It also depends on what champ they are playing. Like against kindred you wanna prevent counter jg and really focus on taking scuttles away early. Or against a lee sin, he should be pressuring lanes early. Countering his ganks puts a dent in his early game. Or Warwick or shyvana take early drags pretty quickly. Ward the drag pit and herald.
Its not that hard. It just takes some practice. Imaqtpie says its the easiest role and theres a bit of truth to it if you take the time to learn how to jungle like a pro.