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ORIGINAL: LafayetteLady

Ok, so you are 21 years old, your profile says you are looking for a sub, and you have been talking with a couple.  What is it they are interested in?  You dominating both of them?  Is this for just play or to develop a long term relationship with the couple?  Is the husband looking for someone to dominate his wife while he watches?

You plan on meeting them in public first, what else is it that you feel you should check?  Are you worried they are going to give you a roofie and leave you blacked out in a hotel room with all your cash stolen?

I'm really unclear as to what you want to be checking.  You have spoken online, and are ready to move on to meeting.  You know to meet in a public place, and seem to realize the first meeting (or two) should be neutral and in public so you all can talk and get to know each other better.  You want to do a criminal background check?  Run credit reports?  Ask them for references from the local kink community you can follow up on?  What exactly are you looking for here?

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ORIGINAL: LafayetteLady

What kind of rules are you looking for?  So the husband is a cuckhold and the wife is dominant and wants you to join them for a play session.

Make whatever rules you want to feel comfortable.  When you meet, ask her specifically what she is looking for you to do.  You aren't the dominant in this situation, just a male body they are using for a scene.  So come up with the parameters that you are comfortable with, and tell them what they are.  If they have interests outside those parameters, you either negotiate with them to come up with a scene you are all comfortable with or you walk away.  This isn't rocket science.

ETA:  and just because you are male, doesn't mean you should forego the need for a safe call or letting someone know where you will be and (generally) what you will be doing, along with the couple's name and address, just in case something should go wrong.