Every club in the Saudi League has strengthened their teams in such a way that it is difficult to distinguish between the big teams and the smaller teams. Ronaldo and Nasser play for the club and some other star players have joined the Al Nasser club so we consider this club as a strong club but outside of that there are clubs like Al Hilal and Etihad who have been performing very well. No match for Al Nasser is easy, every match requires a lot of hard work and proper planning to win. One season Al Nasser is the champion, another season Al Ittihad is the champion and another season Al Hilal is the champion, so there is still competition in this league and that is definitely a good thing.
It's easy, in order to know which are the big teams and which are the smaller teams depend on how much money they have, if they have a lot money, they're big teams and they able to sign European players. There's a competition in this league, but a competition among B class teams. 
If there's an A class team joined the league, they can easily win the league.
It's true, it's very easy to differentiate, by looking at the depth of the squad he have, we can find out which clubs are elite and which are mediocre clubs. On the other hand, even though the gap is still very visible between fellow contestants, I think the competition for the Saudi Pro League trophy will get fiercer every season. Apart from that, former top European players will be a priority for Saudi Pro League clubs to recruit, the aim is to increase the league prestige in the eyes of the world. Many hope that Al Nassr will be successful next season, they will reportedly recruit several top European players to fill the defense line.