I don't think i.Have ever experience a real orgasm, . How do I know for sure, ? What can I do to experience the big o?

You will know. When it happens, you'll know. Many people practice through masturbation; women can have more shame issues to work through, and being alone is often easier at first in those cases. Find what turns you on, and let your mind go there. Try to relax and enjoy the ride, rather than worrying about the destination... And let go of the fear and embarassment that pops in your head. In time, it will happen.
Self exploring. Nobody knows your body the better than yourself. Finding a triger point by self exploring and self stimulation with or without using toys can be helpful to achieve big o. Imagination can be used as spices.
Good info here. this site: http://exploringintimacy.com/about/ and her videos on youtube will provide a wealth of information in a professional and supportive manner. good luck.
Practice and Talk. To have an orgasm with a partner of the opposite sex requires the ability to communicate well. Lots of guys don't understand what it takes for women to orgasm (more foreplay, etc). Women are getting better at helping men with this. Trust me, a lot have no clue. If you can get no cooperation then take that as a sign that your orgasm is not one of his goals, & judge the relationship accordingly.