How your characters treat their horses can be indicative of their personality, morality...and income level.
A horse person can often tell if a horse is being treated well just by looking at it. How can a layman tell? (And can you tell if that carriage ride you're about to go on is okay or borders on horse abuse?)
In other words, how do you tell if a horse is in good condition, starved, or the other extreme - fat?
Let's start with an underweight horse.
These Mustang mares (Image from the Nevada BLM) are underweight - to the point where they've been pulled off the range to let it and them recover (This was an emergency roundup due to drought conditions). The bay mare in front shows the classic appearance of a partly starved horse.
As you can see, her ribs and spine are prominent. Her neck is "inverted", showing a definite concave profile on top. (This can be caused by conformation, but combined with the others, it's a sign that she's underweight.
If somebody's working a horse in this condition they either don't care - or can't afford to care. If they're working one in this condition...they're probably desperate.
This photo is from the great famine in the Ukraine in 1932-1933. (Source: Wikimedia, public domain). This horse is extremely emaciated and literally about to drop dead. In addition to the prominent ribs and upside-down neck, the hip bones are also prominent. (Note that prominent hip bones on an otherwise well-fleshed horse are a sign of old age). Particularly sad is how large the collar is for the horse. It probably fit him once... Tossing in a description of something like this might well make your massive drought seem more colorful.
So, it's pretty much common sense. Prominent ribs and bones = starving horse.
What about the other extreme?
(Image source: sannse via Wikimedia Commons).
Shetlands are chubby. They are not ever supposed to be this chubby. The thick neck, fat deposits across the belly and hindquarters, and lack of any muscle development tell the entire story. This pony is fat, even obese, and headed for health problems as a result. Ponies are more likely to end up overweight than horses.