look at their friends list (Facebook) if they have very few and post about all of the parties (they should have more friends online) or if they communicate with someone quit a bit maybe add them and ask them if they've meet or talked to him/her. also if you have a cyber relationship ask to call and tslk on the phone... or meet up if she declines then that's another indication it's fake. or take their picture and to a web search on it to see if it was taken from another profile or a Google search. there are many many ways to tell.
There is no way, just like knowing the questions asked here are real. That's why you need to be careful on the Internet. Guess you haven't seen the ad with the young women talking to a male friend saying that everything on the Internet is true. Well, it's not! Pictures on Google have been faked.
Sometimes you can eventually tell via what is written. But there is literally no way to be sure unless you meet, preferably some place where you know there are enough people around who would be able to protect you.