>Sometimes, when you have selected the area to travel to, you will get a notice about it being a "Fever" time. This mode adds stars to one or more of the stages, to give you rarer Pokémon to select from. If you get two Fever in a row, it's called Super Fever and will add even more stars to the stages to give you the best options.
>The method of getting this is completely random, but there is a way to see if you're likely to get one. When you return from a previous stage, if there is a rainbow around your balloon in the cutscene, then you have a chance of getting Fever in your next stage, so be sure to make sure you know what stage you're selecting first.
--Serebii on Fever Mode
So yeah, I guess there is no real way of determining the exact chances. I didn't find anything on Bulbapedia either, so I guess that's all I can help you, seeing that there is no real calculation involved. May the odds ever be in your favour.