We have a flat, digital kitchen scale we use to weight Kiwi. We also got our at bed bath and beyond, I think it was around $20 with a coupon. We set him on the scale and give him a small treat so he'll stay on it long enough for it to register his weight. Not sure if it 100% accurate, but its accurate enough for monitoring any major weight fluctuations at home. Only issue we've had is we use it as a kitchen scale too, so theres been a few time we forgot to set it to grams and weighed him in milliliters of floz instead:09: (and he got a second treat so we could weigh him in grams lol).
Edit: We weigh him once a week, unless theres a reason to weigh him more often (such as suspecting he may be sick or eating too much or something).