How can you tell if your horse is uncomfortable with a saddle? besides the bucking

Everything what Lucy said and add that if they swing their tails a lot they have a back pain either from the saddle or something else.
You can also tell if there are white spots along where the saddle sits. The rubbing kills the fur, causing it to turn white.
An uncomfortable saddle will often result in the horse reacting with the following behaviours:
- Being girthy
- Trying to nip at his side
- Reluctance to move forward while riding
- Bucking and rearing
- Rushing rather than being balanced and collected
If your horse is showing any of these behaviours, I'd advise that you pay for an experienced saddle fitter to have a look at your saddle and correctly fit it.
Sometimes it isn't the saddle - it could be that your horse has a stiff back and is tense, therefore it would be useful to loosen him up on the lunge or call a horse physiotherapist.
Hope this helps!
you will feel it, and if you still not sure ask experienced person
Any saddle rubs, if you touch its back and it is really sensitive, tight back muscles etc. I recommend you have a chiro/bodyworker out to see if your saddle is a problem
hope this helped
U do saddle fitting otherwise u wouldn't know until he is saddle sore
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