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it may depend on the computer system.
An easy and convenient way:
On small INTEL-architecture PC-like systems you may boot off a Knoppix live CD (or any other Linux live CD) to get detailed information about all system hardware.
To know what OS is normally running on a box, you have to boot and watch while it is running.
Or you may use some network tools to try to detect the type of OS from the outside (which may often work and sometimes not).