Hi, Kevin-
I'd use a drill bit. Find the size of a bit that fits into the hole. Your bolt may be one size up, but maybe not. Then you have to determine whether it's fine or coarse thread - you have to eyeball that, though. I used to be able to "eyeball" sizes quite well, but I've lost that ability in my old age.
Fine threads are often called "machine threads" in the US, and you'll often find fine threads in holes that are "machined" into blocks of metal like engine blocks and the like. Coarse threads are more common, and are usually (not always) found on nut-and-bold combinations.

I'm still not clear on what's used for weights and measures in England. Am I correct that the metric system is the legal standard now, or are there aspects of British measure that are still official? What now-unofficial terms are still in common usage?
-Joe-