I think you should get them to phone you and then just trust your instincts, if he doesn't sound right, don't do it. In my opinion, there really isn't a 100% effective way to assure complete safety, you just never know, but trusting your "gut feeling" about something/someone is the best you can do, at least during the initial contact. You'll be amazed by how easy it can be to spot a weirdo/timewaster after you've been doing this for a while!