Start by getting ahold of this Special Edition BYO magazine, 150 clone brews, and read the introduction article, Chris Colby goes into a great deal of step by step process in how to figure out clones, or at least get close enough by the numbers to start tweaking test batches.
If you don't have one, you will need brewing software to run the numbers. You'll need to do some digging online or even on labels to find the numbers for ABV, OG, IBU's, SRMS. Often the breweries provide that info on their webisites.
Google is another invaluable tool. The brewery website might not have all the info, but other forums, beer reviews, other clone recipes will all be helpful in beer detective work.
You can order the byo issue here;
http://www.beer-wine.com/product_info.asp?productID=1754§ionID=5 Although I have seen it on the rack or counter of every Homebrew shop I've ever been in.
