Having been an NBC Defense specialist, and attending the NBC defense course at Fort McClellan, and working in related areas: Sarin is a nerve agent, and is a WMD according to the Department of Defense definition. I don't know of any other definition. Pesticides have a very similar chemical structure (both act by the same mechanism, acetylcholenesterase inhibitors)and would test positive for this activity, but as far as I know should be distinguishable by more sensitive analysis.
Thank you for that information which does not surprise most of us but will surprise Galt who is convinced sarin is not considered a WMD (according to some article he will link that is not a Department of Defense or official link, btw).