Not every school will respond favorably with your request to discuss your app. I think it's mainly trial and error as well as luck.
I would recommend going to the school's website and finding the name of the director of admissions. Call the school and ask for that person directly. Some may agree to meet you in person, others may be willing to answer a few questions over the phone. Before calling, work out all the details of your application and have a list of questions prepared. Be respectful of the person's time and remember that they are going out of their way to help you.
Another good idea is to go to any events where admissions staff will be present. The school may have an open house or there may be some other type of open meeting. You can ask your questions in that forum and possibily talk to the person afterwards to discuss more details.
Also, make sure you ask any questions before you submit your application. Once you apply, schools will be unlikely to discuss anything in detail.
If you post your questions on SDN, you will get varying opinions, but hopefully some good advice as well.