Divide the local True Air Speed by the local speed of sound to get Mach number.
To work out the local speed of sound you can (a) look it up, or (b), in metres per second it is the square root of [1.4 x 287 x (OAT in °C + 273.15)]
To work out TAS, divide CAS (which should be about IAS) by the square root of the relative density.
At ISA sea level conditions, relative density=1, and the local speed of sound is 340 m/s, which is 662 knots.
Alternatively, you can do it all on a flight computer.
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