To weigh my bee hives, I use the head, arm, back method. I use my arms to lift the box and if my back tells my head that it is too heavy, then they are good.
some of us old school guys call this the 'heft method' <or the 'oh my god, there goes my back again' method. doable for singles and story and a half... not so easy on anything of double deeps or larger.
when the box fills easy enough to toss about it need feed. when it feels like a brick it is heavy enough.