First of all you will need to talk to them for some time.  Honestly, it's easier in some ways to find out how a person really is online because you don't have all the distractions of a date...getting ready, what to wear, attraction, ect.  But, you must really listen to what they say and be willing to take your time before meeting in person.  I dated online for awhile and trust me ...most of the strange ones you figure out pretty quickly.  There is always the chance that you'll get a "smart" strange one so here are some safety guidelines:

You must have ground rules once you decide to meet.  Always meet in a public place.  Always find out their full name, and address.  Then you can check them out online to make sure they are who they say they are.  Sorry ladies...verify EVERYTHING!  Always make sure a friend knows where you are meeting, if you leave they need to know where you  are going, and notify them when the meeting ends.  Make sure they know who you are meeting and let the guy know that these are the rules.  Take down his license plate number and give it to your friend.  Tell him...I will not take chances with my life so live with it or don't!  Always drive yourself, leave last, and make sure you are not followed.  Once you can verify who the person is, their address, their POE, etc....then you can date without all the precautions.