How would I be able to tell if the belt is sliping ?
It may or may not make noise. If you have a boost gauge hooked up, you may see the boost go high and drop down, even though you are increasing rpms. As the charger builds boost, it requires more hp to turn the rotors. If the belt is going to slip, it will slip as it has to respond to more resisitance from the charger.
From your physics class, you know that the coefficient of static (non-slipping) friction is greater than kinetic (slipping) friction. So, there is less grip once the belt starts to slip, the rotors don't turn as fast, and you get less boost or an observable drop in boost.
To play it safe, you can always just get a belt that's approximately .5" to 1" shorter than what you now have. Make sure you don't go too short or you won't be able to get it on. Belts run about $30, so you may want to wait and see if you need it. With the 2.6 pulley, you should see 8 to 9 psi once it cools down down there.