There are lots of ways to increase the chance of a Critical Strike, but how do I know what the default is? Is it the same for all characters?
The Default critical strike chance of all champions and non-wolf mobs is 0%.
This wasn't true back during the beta, but in present day LoL, it is impossible to get critical strike chance without runes or items.
Notable perceived exceptions are Ashe and Tryndamere, who get crit from their passive abilities, but these buffs adjust upwards from the same base of 0%.