I've always been taught it's more of a feel than measurement. You want the bars to have good even contact all the way down the back. Sit the saddle on the horse without a pad and ease your hand under the skirt and feel for holes and tight spots. Most tack stores here have someone that is pretty proficient at fitting as well.
There are a couple of measurements you can take or even draw a wither template but these can be unreliable. The best way is to put the saddle in question on your horses back and check these things:
1. BARS-they should be in even contact with your horses back in order to distribute saddle & rider weight evenly
2.CHANNEL- ensure this is not putting any pressure on your horses spine
3.GULLET- make sure this is high enough and wide enough so it's not putting pressure on your horses wither
4.POSITION- your saddle needs to placed in the correct position on your horses back so it doesn't retrict his shoulder movement
Hope that helps
Would just like to add - if you are unsure about correct saddle fit, it's always better to get a professional to come out and take a look. It's much easier then getting an incorrectly fitted saddle and having to deal with your horses 'training' issues later on!