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If you searched the web you probably got a million hits on "flow sensor" or "flow measurement" or "flow meter"
There are countless manufacturers of this kind of equipment and countless ways of measuring the flow of a fluid. Try "flowmeter.com" for an introduction.
For ideas on crude, hobby type methods - share more about what you are trying to do. What the fluid is, temperature, pressure, etc. Is it in a pipe, channel, river, etc. Reliability and accuracy may be important.
You can measure flow in a variety of ways and methods. Some being simple and other complex costing a $1000. You'll have to explain more on how much you want to spend, accuracy and what kinda output you want the sensor to give? Industry standard outputs are 4-20mA, 1-5vdc, or even as a proportional pressure.