Take the total amount of GPH from your power heads, filters, etc, and add them up. For instance, if you have a powerhead that does 225 and a filter that does 400 GPH, you would have 625. Then, you would take that total number, and divide it by the gallons of your fish tank.
625/36 = 17.36 turnover per hour
17.36 is how much turnover your tank is getting. You want to shoot for 20 for a reef aquarium. The more flow the better.