Screening is an art not a science. Generally you can screen the names, phone #'s and e-mail addresses using google, bing, social sites, etc. Sometimes this can really help. Also go with your gut feelings, if something seems off, it may very well be. Ask questions and see if the answers make sense.
A bit more info about you and your VR might be helpful. Your country and town, and your VR's country and town.
Thanks for your reply. Our VR is in Ireland (we are currently living in the States) and could have guests from just about anywhere. The person I've received an inquiry from (2 emails now) did not indicate what country he's from or provide me with anything else (like a phone number). All I have is a name, email address and the potential dates we've been discussing. I have googled him and looked him up on FB, but unfortunately there are a number of people with that name, so that isn't helping me a whole lot...
I'm still new to this, so I'm trying to figure out a good procedure. When I've rented in the U.S., I've met people face-to-face and I ask them to fill out a rental application which I use to check references. But, this is a different process...I'd like to get an idea of who I'm dealing with before I sign a rental agreement!