Oh Heck, this is a really difficult one to answer without being able to look at the saddle on the horse and see for yourself and learn what to look for.
The very basics are,
First look at the channel or gullet, is it well designed? you should be able to get at least two fingers width down the length of it, any narrower especially at the back and it will pinch the spine.
Check to see if the tree is broken, this can be overlooked by even the most experienced rider. Check it at the front arch and along the length. Hold it with the cantle in your belly and grab the front of the saddle either side of the pommel, see if there is any flexing in the pommel. If you hear squeeking or crunching get it looked at. Next grab the pommel with the cantle still in your belly and see how much fles there is towards you. There sould be a bit, but it should spring back (if it it a spring tree!) Again if you hear squeeking, crunching or it does not return to it's original shape get it looked at.
Look at the panels, look for lumps and dips in the stufing.
When it it on the horse it should look as if it is part of the horse and not perched. When the horse is walking with the girth done up it should not swing from side to side at the back. As a very general rule, with no rider on board the cantle should sit very slightly higher than the pommel. It should not tip forward or back.
There should be sufficiant room all the way down the channel to clear the spine so that the weight of the rider is carried on the muscles either side of the spine and not intefere with the bones of the spine in any way.
As much of the panel as possible should be in contact with the horse's back.
Make sure the shoulder can swing freely and you don't get a bunching of skin in front of the knee rolls as he walks as the shoulder comes back.
These are a few BASIC rules, there is a lot of give and take with them. But as a rule if the horse is going well, the spine is clear and you are comfortable, don't worry too much. This post has gone on too long as it is!!
