I tried searching online for a how-to guide as well and like you ended up empty handed :(
I think your best and most reliable option is to visit the nearest Louis Vuitton shop to have it authenticated there. Although I have read Louis Vuitton will only give verbal confirmation that the item is authentic but no written document will be provided.
I found someone of ebay who offers Link hidden: Login to view
Also, you might find the description of the metal parts of this Link hidden: Login to view
helpful in discerning if your keychain set is the real deal.
Still my first option is visiting your nearest Louis Vuitton shop.
Hope this helps!