Confusion over drips after using toilet

Sometimes after using toilet to unrinate and doing istinjaa I can feel drop along my leg or fall below my kness to the calf area. This make me very much confused if it is urine or water from drop after istinjaa. I have tried to sit down for a while after istanja but it happens. This causes a confusion if I am clean or not and that leads me to delaying of prayer. What should I do?