Yes, she knew all right! If an angel appeared to YOU and made this pronouncement, wouldn’t YOU Know?
She also knew there were serious personal sacrifices and future sorrows (Simeon in the Temple warned her, for instance) and, being without the stain of original sin, she may also have had some communication with God in prayer or sleep or who knows what else that we are not privy to. What I often wonder is: how did she experience the horrific torture of her beloved Son, KNOWING Who He was? She couldn’t have been angry (although there is righteous anger, even Jesus demonstrated it at the Temple with the money changers); she must have been confused because, perhaps like others, she thought the Messiah would be powerful and conquer (but then, with that private communication with God, perhaps she intuited or was instructed this was not so???) How did she FEEL watching all of this: as any mother would. The torment of watching your beloved child suffer for something he/she did not “deserve” (from our limited perspective) is EXQUISITE. Not being able to DO anything (remember: at that time women were little more than chattel) she could DO nothing except stand and watch. She would have to show as much strength and emotional restraint as possible for HIS sake, absolutely. But the interior horror was enormous, the stuff of nightmares and lifelong horror. When He rose, imagine her incalculable joy and perhaps at that point she had been given the “truth” of this and knew He would rise. Or perhaps it came as a total surprise. Do we know? I don’t think so. What we do know is that she followed his every step, she suffered enormously, she remained calm for His sake (she must have done, any good mother would) but He knew the agony in her heart (which increased His own agony because He so loved her). She stood at the foot of that rough honed, sharp with bark, tree trunk and every single thing He suffered she suffered with Him, for us, and now suffers with us when we lose our children.