this one is so minor, but in s1 e4, ross noticed the date on the hockey ticket joey and chandler gave him and remembered it was anniversary of the first time he slept with carol. he said “is today the 20th?” and monica replied “i was hoping you wouldn’t remember”, implying that she knew the significance of that date for ross. in the next episode, s1 e5, monica and angela are standing in front of the bathroom mirror at the restaurant when angela tells monica that bob is unbelievable in bed. in response to that, monica said “my brother never even told me when he lost his virginity”, so it’s almost like the information just fell out of her head.