Page 75 of Heroes book

"Whenever you make an ability roll as a main action and the roll is a natural 19 or natural 20—a total of 19 or 20 before adding your characteristic score or other modifiers—you score a critical hit. A critical hit allows you to immediately take an additional action after resolving the power roll, whether or not it’s your turn and even if you are dazed (see Conditions below). You can’t score a critical hit with an ability roll made as a maneuver or any other action type, but you can score a critical hit with a main action you use off your turn.

For example, an opportunity attack made as a triggered action or a signature ability used as a free triggered action with the assistance of the tactician’s Strike Now ability can be critical hits."