In D&D 3.5, one scored a critical threat, and then had to confirm the crit by hitting the monster's armor class again. In D&D 5th edition, it does not explicitly state that crits have to be confirmed (to my knowledge). Thus I have been operating under the assumption that crits do not have to be confirmed. Can anyone provide insight into this matter?
Correct, critical hits do not have to be confirmed. Any 20 on an attack die is a critical hit. (Fighters of the Champion archetype eventually score critical hits on rolls of 19, and then 18, also.)
Rolling a 1 or 20 ...If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target's AC. In addition, the attack is a critical hit. (PHB p194)
Critical Hits When you score a critical hit, you get to roll extra dice for the attack's damage.... (PHB p.196)