We set up a weighing machine a few years ago. It involves a spring balance reading beyond what you expect your sheep to weigh, so for ours it's probably 100kgs (not going out to look

) - the spring balance is the expensive bit but we already had ours. Hang it somewhere secure, either in a door frame or build an A frame outdoors, construct a sling for the sheep such as a sack tied at each end and passed securely under the belly, then hook it all up. Obviously you have to make sure the sheep doesn't wriggle but after a few you get a feel for the true weight of all your stock so it's not something you would do for every single one.
Another way is to weigh yourself, then pick up the sheep, step back on the scales and subtract the difference - not many scales weigh high enough for that to work with a mature animal, even if you could lift it.
You could perhaps also buy a very expensive weighing crate but for a small flock it wouldn't be cost-effective.
I have a photo of our set-up if anyone wants to visualise it.
More usually when judging whether your lambs/hoggets or older sheep are ready for market, you would assess their condition score, as this is more relevant to readiness for slaughter than their actual weight which varies greatly with breed and age.