You could also try using a scale - however - you would need the right kind of scale and a cooperative duck! I do the duck-lift method, myself - I pick them up and check - along the lines of the chart needlessjunk posted for you.
I know what 5 pounds feels like, I am pretty good at guessing small weights, so that's what I do.
I know what 5 pounds feels like, I am pretty good at guessing small weights, so that's what I do.