That is actually a good question! I don't know if I'm 'scientifically accurate' but I check behind the girth and right where the close contact saddle ends. If I can feel the ribs we are good... if I can see them then I consider the horse is a little skinny (understanding that all the other BCS points suggest the same thing).
I run my fingers along the ribcage area, where horse is widest "sprung" on their barrel. That area seems to have the least fat cover, so I can find ribs quicker, if they are findable.
If I can't find them even there, then I probably won't find any ribs elsewhere on the equine. Too fleshy or just plain fat! Guess then he would be getting rations reduced a bit, or shorter grazing times, with longer dry-lot time. He doesn't need extra calories, has been using them too well!
I am not saying this is scientific, just a practical, easy way to check rib coverage for being too fat.
Of course you do need to look at the whole animal, see if neck is cresty, fat pads above the tail head or down along the belly in front of the mare's udder, which are obvious signs of carrying too much weight or a body problem that needs attention.
Since none of my equines have body problems, the rib test is working pretty well for me at this time. We want fit athletes, not hog-fat horses who are carrying weight they don't need. They can't turn, run well, and it is harder on the legs, to carry excess weight. I want a flash of ribs in big bending turn, like a good racehorse has. With hairy coats, you have to do a hands-on to the animal, so you don't miss seeing or even FINDING the ribs with your fingers or discovering fat pads, covered by long hair. Blanketed animals need to have blankets removed every couple days, get groomed, have your hands on them, to access TRUE body conditon by feel.