A natural 20 on an attack roll is a critical hit and is always successful. [AG 10 and also AG 444]
Thanks! It's a lot of pages, I must have missed it.Critical hits follow from the number rolled on the die:
Ergo, expertise dice, which change the overall total of the roll but don't change the number rolled on the die, don't determine whether an attack is a critical hit or not. (Unless, of course, you have some special ability that says they do.)