How do you find the cap on medical insurance policy?

The cap is generally called the Maximum Out-of-Pocket. This is the most you pay in any year before the insurance company pays for 100% of covered expenses. It is the 'covered expenses' however that can get you. Many plans put limits on some coverage or specifically exclude some items from the MaxOOP. A good example is chiropractic of physical therapy. It is very common to see a specific number of visits limitation in a policy. Consequently even if you have exceeded your MaxOOP if they limit your physical therapy to 12 visits per year, any visits after that are on you.