CPU as in processor, is's clock speed is measured in GHz - Giga Hertz.
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By benchmarking. There are many different standard benchmark programs out there that can be used, but as each one measures something different about the performance of computers they must be carefully selected.
Never simply compare clock speeds. One computer may run on a very high speed clock, but be noticeably slower than another with a more efficient architecture and/or implementation running on a slower clock.
The best benchmark is the actual set of programs you use the most, but there may be problems even here as they may not be supported on all the computers you want to compare.
CPU speeds are measured ussually by Instructions per second. Ussually this value is high so we use Million Instructions per Second (MIPS).