To score a critical hit in 4th Edition D&D, do the following:
Roll 20.
Simple enough, right? Just one number to remember. And more importantly, just one roll.
Yes, the confirmation roll is gone. So why did we get rid of it? Because we, like so many players, had rolled crits only to have the confirmation roll miss. And we didn't like it. We don't think that many people did. (I look forward to reading the posts of people who disagree.) Having one roll is faster, and it's more fun. It keeps the excitement of the 20, and ditches the disappointment of the failure to confirm.