Can they keep laying? Sure. I have several hens who are 5 1/2 years old, still laying several eggs each week, some who are 4 years old (one of those is my very best layer, still laying 6 eggs per week), some 3 years old, etc.
How would you know? To know which ones are, you might want to check their vents to see if they are wide and moist, not small and tight, or you could spy on them on a couple of days off to see or set up a coop cam.