It's more the width of the pubic arch of the pelvis, rather than the hips themselves. Though generally they're related.
Short of an x-ray to measure the distance between bones, which could then in theory be checked against tree-rail position for different saddles, the most practical way is to sit/ride on as many different types as possible.
But - it can sometimes be difficult to tell if a saddle is comfy or not straight away. One customer last week was uncomfortable riding her Bates, and since these have a 'flat plate' synthetic tree it was difficult to see what was causing the problem. It seemed to be the way seat and skirt were stitched together - the welt was raised, and that meant there was a hard 'lip' between two softer sections of leather.
The Bates/Wintec/Collegiate Convertibles are all on the same two trees (flat seat and deeper seat.) They are too narrow for me. Many close contact saddles, especially the old original plain flap ones, have a narrow twist. They were meant for 2-point, not sitting.
Thornhill 24k and HDR Advantage and Pro have wider twists. Not sure about the Rivella and DeRivel or whatever the top-end is. The current Crosby Prix with the wider padded seat is also very comfortable for me. I liked the Stubben Edleweiss CS from a few years back, but the Artus didn't feel the same.
Most AP saddles have a moderate to wide twist. Dressage saddles vary.
M Toulouse saddles are described as having a wider, moderate twist by the company. They're generally pretty nice saddles too. One of those might be worth checking out or taking out on a test ride. I really like mine.
Duett is another moderate to wide one, if it fits your horse. There aren't very many ways to get a narrower twist on a wide or XW tree saddle except to build the seat up higher. Some people complained that the Duetts (generally bought for wide to XXW horses) had an uncomfortably wide twist. I owned an Allegro 17.5" 35 cm, and the deeper cutback head plus deeper seat makes that model ride a bit smaller.
The only saddles I've ridden in that I've found physically painful are prestige, wintec and Windsor.
With the prestige, it depends on the model because my one student has one and it's fine for me, but others I've sat in hurt.
Windsors have a very narrow seat and I find my seat bones actually sit off of the seat on either side leading to hip pain.
I find I actually prefer a "mans" saddle and ride in a KN, which has quite a wide twist. I'm not a big fan of Schleese, and find them a bit hoodooey. Yes I've had a schleese, never fit me or the multiple horses I got it "fit" to. Any so called "adjustable" or "custom" saddle I am very leery of... Especially the "custom" saddles which all have the exact same tree. If I wanted an adjustable gullet saddle I'd have a wintec. Posted via Mobile Device