FRISCO, Texas — Dallas Cowboys quarterback Dak Prescott has no boxes left to check. After missing the past five weeks with a fractured right thumb, Prescott’s return to the starting lineup for Sunday’s game against the Detroit Lions is a certainty. “I am,” Prescott said when asked Thursday if he was starting against the Lions. “I think anyways,” he followed with a laugh. Prescott been medically cleared. He’s practiced full and looked very good throwing the ball with velocity and on a number of routes. He’s taken the majority the first-team reps in practice on Tuesday and on Wednesday and has b…