Cheapest and easiest is penetrating fluid. Belts squeak from glazing which is very hard stiff rubber but smooth, so it skips on the smooth metal pulley. A new belt is normally soft and has more friction with the pulley so it slips less or almost zero.
The penetrating fluid doesn't fix the problem and it could make a non squeaking belt start squeaking till the oil wears away, but you will hear a change if the belt is the cause. It's like rubbing your hand over a mirror and you get the smear sound, throw a little oil on your finger and do it again and it glides with less effort. If you wash your hands to not have the natural oils on your fingers, your finger will have more friction and make more noise when it slips.
Remember, a tiny bit goes a long way, don't soak the belt or you might have very bad slipping pulleys which will be worse than a noisy belt. I did this first hand long ago with out thinking and my water pump runs off the back side of the belt. The water pump ran slower and the engine ran a little hotter than normal because I soaked the back side of the belt. It made the squeaking go away though for a few miles. Replacing the belt fixed the belt in my case, inspecting it showed a shiny hard surface on the belt.
Also did a quick search on belt dressing since I've never used the stuff. Seems like it's not advised to use on modern belts.
http://www.daycoproducts.com/why-bel...t-a-noisy-belt