1. Don't be afraid to measure.
2. Keep a "morgue" of photos of the animals for reference.
3. When it comes to size, a "handful" is appealing.
4. The real secret is to participate in the "being" of the calf -- not just the way it looks to the rancher or other onlookers, but what it feels like to be a calf.
That said, my customers are likely to be ranchers, so I'd better take special care to be accurate. White is Charolais and will have a pink nose. The "Hereford" in the back is not quite a dark enough red, but that's the color of my FIMO. Next is a black Angus. I think I'll try a newborn.