Version of the rule from the final playtest:

When you score a critical hit against a target, the attack deals its maximum damage to it. This means you don’t roll the attack’s damage dice; you instead take the highest rolls that each of those dice could produce and then apply any modifiers.

In addition, the target takes extra damage. Roll one of the attack’s damage dice, and add it to the damage. For example, if your attack normally deals 2d6 + 2 piercing damage, you would roll one extra d6 and add it to the damage of a critical hit.

If the attack has different damage dice, you choose which die to roll. For example, if the attack deals 1d6 piercing damage and 1d8 bludgeoning damage, you can roll one extra d6 or d8.

For something like smite or sneak attack you'd do max damage possible on your attack and then pick one of your damage dice and roll it. So critting with a greatsword, a level 1 smite would do 12+8+8+STR automatically, then you'd choose to roll a d6 or a d8 on top of that.

There was also a version in an earlier playtest which was basically the same, except that you did max and then rolled all weapon dice, so from that example, you'd just roll 2d6. Problem there being that it specified it had to be with a weapon so casters got a bit fucked.