Consider water flowing in a pipe, and I don't know the direction of flow. How to determine the direction of flow? My approach- I can use a pitot tube but for that I have to drill a hole in the pipe and I can't use this approach for fluids whose leakage will create a severe problem.

What is the simplest way to determine the direction of flow?

Consider water flowing in a pipe, and I don't know the direction of flow. How to determine the direction of flow? My approach- I can use a pitot tube but for that I have to drill a hole in the pipe and I can't use this approach for fluids whose leakage will create a severe problem.

What is the simplest way to determine the direction of flow?

Edit- Is it possible to determine the flow direction without using gauges or any specific instrument?