Something I always do is search their email address, name or any other information on Google. Doing this usually clarifies who the person is and how you should respond to the offer.
To my previous comment that I never respond I must add that, in a couple of cases, after receiving emails of a doubtful nature and using Google to search information about the sender, I have responded. In one of those cases, it was a nice sale.
I received an email from a guy in Malaysia with a $300 offer on a domain I bought on Ebay for $9. I responded with a $500 counter offer and he agreed. A few days later I received the money via Escrow.com and transferred the domain.
In short, analyze and do research using the information contained in the email you received before responding.