Well I would assume you would look up the vegetables that you are adding to the juice. I am not really familiar with juicers I usually like to eat my veggies whole, or blended with fruits. I know that WHfoods.com has a great index of vegetables and gives their vitamin and mineral composition. That's what I would suggest doing but I am not sure if it would work for what you are attempting to achieve.
www.nutritiondata.com is a massive U.S. government sponsored database of whole and packaged foods. If you sign up (it's free) you can use the site's recipe feature to plug in the fruit and veg you use (and amounts) for your juice.