Using other posters’ suggestions for blanketing, start there.
But since each animal is different, it’s always a good idea to check on how your horse is tolerating the clothing. A horse dressed too warmly will feel sweaty or damp under his blanket. One that’s not dressed warmly enough will be a little more ‘high’, and when you pop your hand under his blanket, he won’t feel warm. This is all an issue of personal perceptions, personal preferences, and it will change as your become accustomed to ‘reading’ your horse.
Take your already-warm hand, and feel, palm down against your horse’s shoulder under his blanket. Be sure your hand has been kept warm before feeling your horse up. If your hand is cold, by not wearing gloves, or having recently washed buckets, or bad circulation or what have you, you won’t be able to know accurately how comfortable your horse is.
Your hand should feel the warmth radiating off his body. You should immediately feel relief from the cold air/ weather. Hold your hand there for several moments. If the back of your hand starts to feel ‘colder’, you may realize horse is under dressed. If you feel moisture/ dampness, horse is likely overdressed. Over time you’ll learn to gauge a too-hot or too-cold horse.
Then, (careful now… don’t get kicked)… I like to feel between their hind legs. Feel any moisture? Sweat is BAD. Adjust clothing accordingly. I also feel under their blanket on their haunches. This is a great place to get a grasp on if they’re too cold, but unlikely you’ll feel sweat here.
Some people thing that by feeling the tips of a horse’s ears you’ll be able to tell if they’re dressed properly. This is ridiculous, IMO. I can be sweating bullets under my parka and my ears will be near-frost bit. One has no bearing on the other.
I know this doesn’t help someone who’s only got access to one blanket and is shopping for more. But once you’ve got a bunch of horse clothes from which to choose, you’ll use these guidelines nearly every time you blanket.