How do you determine that your hardware is 64 bit capable when there is no operating system installed?
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Look in the BIOS, they often display a summary of the installed hardware including the CPU model, you can then search that on the web and see if it's 64-bit compatible.
Alternatively, if the computer is of a known brand and not a custom-built one, you can search for its model and you'll find a lot of info on it, like the CPU that's in it (then do what I said just above).
Unless your hardware is ten or more years old, it's virtually certain to be 64-bit capable -- all Intel and virtually all AMD x86-compatible processors made in that time frame have been 64-bit. Most processors for Android devices are 32-bit, but since most of those machines can only run Android anyway, it's less of an issue with them (and if you have a single-board computer like a Raspberry Pi, you'd probably know that from buying it or can look it up by the device model designation).
If your processor is any non-ancient flavour of x86 processor, you can use the CPUID instruction. On modern processors, you can use it to find out if your CPU is 64 bit capable. On older processors, you can use it find out if you can ask the processor whether it is 64 bit capable; if you can't ask it then it isn't. If you look deeper into Intel's and AMD's manual, there is code running on even older processors that allows you to check whether the CPUID instruction is available; if it isn't then again your processor isn't 64 bit capable.
You shouldn't even need to turn on the machine, look for the model number on the back of the case. Unless it is truly a white box PC, you should find a model number or even a serial number. Search the web or call the manufacturer.