Making wudhu with trouser that might be napaak

Question is totally based on doubt. When I stand and pass urine and if im in a doubt that few drops have touched my trouser but this is a doubt as my trousers are dark and sometimes drops are not even visible if they have touched or not. In this case of doubt what is my duty?
Im looking for solution as half of my day goes around at work and im always in doubt if my clothes are clean. I want to stay and keep myself and my clothes totally pure before praying..
Please advice!!