How to measure RPM of any rotating shaft?

can anyone plz tell me how can i calculate the RPM of a motor or anything that is rotating???.... tachometer is one solution has anyone got another solution???

digital tachometer is the only solution that i know of, its fast and very reliable in this day and age, any other method would require mechanical contact with rotating device this means precision mechanical device would cost 10 times more than a simple digital tachometer which you could grab form rs9000 to as high as rs15000+


Hall Effect sensors.

Connect a dynamo and an Mulitmeter to measure the Amps