There are two aspects of fitting the front of a saddle - the angle of the inverted V, and the horizontal width between the points of the V. If the angle is right, but the horizontal width is too great, then a wither pad will work fine. If the angle is off, or the horizontal width is too narrow, a different saddle is the right answer.
Also, there is a difference in what the bars do after they leave the gullet. Some are set wide and flat, others are closer together or bend a lot. And some go a long way back, and others do not.
In the end, I don't know of a good substitute for trying the saddle on the horse. If it is from a reputable Internet store, you can return the saddle if it doesn't fit - but you'll be out some money. A local tack store will usually let you try it and return it. I sold a used saddle and told the woman I'd hold her check for a week, and if it didn't fit, return it. That could have burned either of us, but we were both honest. The next day, I got a call telling me to deposit the check because it fit fine.
Here is a link that discusses the problems: Avoiding the Withers - Using Hand Hole Height and Gullet Height Measurements