… They will measure the width of your sit bones. From that measurement you can insure that you are riding the correct width saddle. …
A top end bike shop will allow you to return a saddle that doesn’t work for you.
Trek/Bontrager also a saddle width bench sizer. Based on that their saddles are color coded in at least four road models. Trek has a full 30 day money back guarantee where you can bring the saddle back for a full refund for whatever reason. (I used to sell Trek, don’t anymore, thought it was a good policy).