Flow mesurement

Portable clamp-on ultrasonic flow meters are commonly used to verify existing flow measurements, if the service is clean water this would be your ideal solution.

Since you are only trying to confirm flow, there are two ways to do this easily.

First- use a paddle-type flow switch that will be triggered by the flow (you can pick the flow needed to trigger)

Second- for non-invasive method, use a differential pressure switch across some part of the system- a valve or coil or whatever. You can set the differential that activates at whatever delta-P is triggered by the device- small delta-P for a valve, larger for coils, etc.

None of the above are easy to implement if only because they will include modification of piping. Neither is dp flow measurement non-invasive.

why not use a simple microphone? water flow will always generate some specific noise you can pick and listen to. this could be the simplest solution since you do not ask for a quantification of flow only for its presence.

That works OK. CMU recently developed a low cost, low accuracy flowmeter something like that. I don't have a link to it, though.

There are devices that measure the flow using ultrasonics. Device is placed on outside of the pipe. Must meet a couple of conditions. You need a section of pipe that is straight for at least a couple of feet or possibly longer depending on pipe size to ensure fairly laminar flow. Also need to have some contamination to bounce the sound off of in the fluid flow. I have tried it on hydraulics and it was not successful as the oil was too clean. I believe you can have issues if too fluid is highly airated as well.

Out put is in velocity and if ID of pipe is know you get a fairly decent reading on flow rate. The brand I use escapes me but I just googled ultrasonic flowmeter and you will get a ton of hits.