It's been very evident with him for three years already.
I remember when they were just starting with finger foods and to feed themselves (I remember those days when I perfected the art of dumping a kid in a bath, washing and out in less than 5 minutes - quicker than trying to clean food everywhere!)
Anyway, he'd always grab for the food or spoon with his left hand. I would set down the bowls with both their spoons on the right and he'd swivel the bowl til the spoon was on his left, and then dig in.
Remarkable actually.
He's always held scissors with his left hand, cups with the left hand, etc.
I bought him these left-handed scissors about 6 months ago and he loves them. Poor thing :) He was always very good at cutting but now he's amazing. It looks like my cutting (okay, that's not saying very much - I am soooo not a perfectionist "done is better than perfect")
So tell me about your twins especially but I also want to know with your singletons, when you noticed their handedness, and how?