Is there anyone in canon who was stated to be able to tell Fred and George apart? Consistently, and not by randomly guessing.
Ron can tell them apart: in Harry Potter and the Deathly Hallows chapter 22, Ron recognizes Fred from his voice through the radio.
Harry can tell them apart at least in Deathly Hallows, where in chapter 31 he recognizes
Fred before he dies in a duel,
but it might be easier at that point, because George has one of his ears hexed off by Snape.
I believe their mother Mrs. Weasley can tell them apart, see Chamber of Secrets chapter 3. However, Fred and George try to deliberately confuse their mother, as seen in Stone chapter 6 and in Hallows chapter 5 when George says, after his injury:
‘You'll be able to tell us apart now, anyway, Mum.’
I opine J. K. Rowling could tell them apart as well.