You cannot judge a horse's weight reliably by eye alone. A thick winter coat hides ribs, a grass belly can look like fat on a horse whose topline is actually poor, and 'he looks fine' has concealed both creeping obesity and quiet weight loss. A healthy-weight judgement comes from your hands, a scoring system used the same way every time, and a weigh tape for the trend.
What your hands should find
Run your hands over the key areas in turn. The ribs are the anchor point: on a healthy horse you can feel them easily under a smooth covering when you press gently, but they do not show as clear ridges when he stands relaxed. The neck should meet the shoulder cleanly without a hard, raised crest, and the crest itself should feel supple rather than solid. Behind the shoulder and over the loin the surface should feel flat and even, with no spongy pads building, and around the tailhead the bone should be covered but still findable.
Muscle is not fat, and the difference matters
A horse can carry a big belly and still be under-conditioned where it counts. Look along the topline: a healthy horse has muscle filling either side of the spine so the back looks flat or softly rounded, not peaked with the spine standing proud. Fat, by contrast, collects in patches, at the crest, behind the shoulder, over the ribs and around the tailhead, and it feels soft or spongy where muscle feels firm. Judging both separately stops a fit but lean horse being overfed and a fat but unmuscled one being declared fine.
Score him the same way every time
Use one recognised condition-scoring scale, either the nine-point or the nought-to-five version, score each area, and average honestly rather than scoring the best-looking part. Most leisure horses in light work sit healthily around the middle of the scale; hard keepers run a touch lower and native good doers fight a constant battle against the top. Weigh-tape at the same time, same place on the barrel, pulled to the same snugness, and treat the number as a trend line rather than a true weight. Score monthly in summer, and more often through spring grass and winter, when change is fastest.
When the picture needs a professional eye
Sudden change in either direction, a crest that hardens and thickens, fat arriving in uneven patches, or weight falling away despite a full ration are all reasons to involve your vet rather than adjust feed and hope. Rapid, unexplained change is a health question first and a feeding question second. Recording each monthly score and tape reading in your KinsLe Pet Passport builds the history that lets your vet judge whether this horse's weight is stable, drifting or genuinely wrong for him.