1. Look at the brewers website for any info already given to you.
2. Contact brewery and ask them. Some will divulge a lot, some not so much.
3. Search online for clone recipes or info.
4. Drink the beer with the information you now have and finalize the recipe.
I actually own a book called "Clone Beers" by Tess and Mark Szamatulski that has about 200 clone recipes in it. That being said NCBeernut has some good ideas. I never thought of calling the brewery but I do go to the website always, and in the case of Sam Adams I toured the brewery and asked questions.