if you put them on the ground it's easy to estimate their weight with a fairly accurate result.
You need to use the "Heart girth" and that is measured using a string or fabric measuring tape. Put it directly behind the front legs, warp it around tight and read directly behind the shoulders.
pig weight = 10.1709 x Heart girth (inches) minus 205.7492. This should get you to +/- 15 lbs.
There are other formulas out there using length too, but I've found this one to be pretty quick and easy to do, especially with smartphones and excel handy (if you keep a log)