There is no surefire way to tell if someone's married. There are signs, but ultimately you won't know for sure.
In terms of safety, meeting them for the first time at a neutral place (e.g. coffee shop, pub etc) is best. Don't ever let someone pick you up, don't get in their car until you're comfortable with them.
I always arrange with a friend to call them an hour into the date to let them know I'm okay. If they don't hear from me then they're to try to call me, if they can't get through then they ring the venue where we're meeting, if I'm not there then they call the police. (This has never had to happen BTW!)
I also arrange a code word so if I text my friend saying "Having a SUPER TIME" it actually means "Please call me now and fake an emergency because this person is a twat"
For a first time sex session I prefer to book a hotel because if something does go wrong it's more public, whereas your home or their home there won't be anyone in earshot. You can book daytime hotels if you look around airports as lots of people book stopovers.
I would not invite anyone to my home until I'd seen them several times. So basically the same level of safety as a "normal" date, who you wouldn't invite round immediately either.
BTW a book I'd recommend for anyone dating (hookup or otherwise!) is The Gift of Fear. It really helps you learn to trust your instinct and listen to your gut reaction, and squash that little voice that says "I don't want to get in his car... but that seems rude, so I'd better"