Ihave had people tell me thier child weighed 39 lbs and when I get there it is obvous they do not. That particular child only weighed 33.5 lbs. I now always have a scale.
I know what Piper weighs, and I've been uncannily accurate lifting other children and comparing them to her. As she gets closer to the 40 pound mark it'll be harder to tell, but for now I can just lift them and see how close they are to Piper. I'm usually within 0.5 to 1 pound.
Wendy