Hello,
Hilarysmum's advice is what we did at first until we got the eye for this. One tip is that you should just be able to feel the ribs with gentle pressure, if you can clearly see the outline of them they are too thin and conversely if with gentle pressure you can't feel them then they are too fat. In theory you could use a sheep weighbridge - don't buy one though, most large flock keepers would have one so I'd use the "neighbour" system.
Cheers
Gavin