Here's one thread to get you started. There are several. Search the forum for measuring sit bones, cut-out test, etc. if you want to read more.
Or, Specialized and Trek dealers have a pad they use for measuring.
Some LBSs, but not all, will let you try a saddle for a certain amount of time; it never hurts to ask. There are also some places where you can order saddles online with an option to return them.
I think that just sitting on a saddle on your bike at the LBS might eliminate some that definitely won't work for you, but IME I really need several rides, including some long ones, to know for sure if a saddle is going to work. Plus, any time I put on a new saddle, the rest of the fit changes and I need to make some tweaks before I can really form an opinion. Some people get lucky and find a saddle that works pretty quickly, for others it can be a long and expensive search.
If you see a saddle you want to try, don't hesitate to post an ad in the For Sale/Wanted section, someone may have one that they didn't like.
Good luck!
Last edited by OakLeaf; 11-27-2009 at 06:43 AM.
Speed comes from what you put behind you. - Judi Ketteler