Measure the frequency of the sound using a microphone and a frequency meter then divide that number by the number if cylinders.Note: If the mic is too sensitive you may get a number that is 2,3,4 or more of the actual rpm so adjust the mic so that it just barely picks up enough sound to drive the counter.
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what kind of software can i use
You need an optical RPM counter which any good RC supplier will sell, but going back to your previous question, there is an absolutely brilliant guide to motor and prop selection on the RCGroups site. This guides you through the whole process starting with your model wingspan, wing surface area and weight, and by using the WebOCalc program, will help you determine the best prop and motor setup to use, as well as giving you the expected speed and handling characteristics of your model and the current rating requirements of your batteries and ESC.
Most RC shops or web shops will sell a none contact tacho. there is really any other way.
Here's the first thing that came to mind. Why not take a IR emitter/detector pair (set up so the prop will break the IR beam but not hit anything), hook it up to an arduino or an oscilloscope to count the frequency and divide the average frequency by the number of blades.