Identifying PC 64 Bit Capability

Is there some unambiguous way to positively identify whether a PC is 64 bit or 32 bit?

I have inherited a couple of what appear to be perfectly fine PCs that had 32 bit XP OS originally installed. I understand, but can't remember where I may have read this, that in the XP era some 64 bit machines were shipped with 32 bit OS installed as 64 bit OS were not yet reliable.

How can I unambiguously determine whether the machines I have are 64 bit or 32 bit?

Thanks, in advance, for advice, counsel, instructions, or whatever on this subject.

You need to know the cpu and motherbaord

There's also GRC's "Securable" utility, which should be able to tell you. (This assumes the machines still boot a working Windows.) I used to use that years ago to tell which XP machines could be upgraded to Win7 x64 and which had to stay on x32.

Gibson Research -- I hadn't thought of looking to see if Gibson had anything. Whatever, 'Securable' worked well. Intel's 'pidnu46.msi' also works. Thanks to all.

A really simple rule of thumb is that, if the CPU is at most something like 10-12 years old (save for few exceptions like cheap netbooks) it's almost always 64 bits capable. Pretty much every computer for quite a few years is, really.

But for a more serious test, I would simply try running a 64 bit OS on it.
Try to boot it with a 64 bits Windows 7 installation disk. If the installer runs, then it's 64 bits. If it's not, it'll immediately give some error telling that the computer is unable to run the system.