Slugging a bore consists of pushing a slightly oversized, lubricated soft lead bullet through the barrel so that the groove diameter can be measured.  It's important to know this so that you can choose a bullet diameter that fits the barrel.  It won't tell you the caliber, if you are referring to what cartridge case the chamber takes.  For that, you will have to do a chamber cast using a low temperature material such as Cerrosafe (available through Brownells) and then measure the casting.  Next step is to compare the results you've gotten with published dimensional data, then get a mould, brass and dies.

David
Montezuma, IA

Thanks to both of you,
I knew some one would know how to do the above. I have already 
ordered some Crro-Safe.
Thanks again, Ray

Sure, I'll give it a try, if he doesn't mind editing it! I'm not really the journalistic type!

The method I have used for years is that described by the late Claude Roderick in Precision Shooting back in the mid 1970's.  Though not a lot different from what has been previously posted, it might be worth your persual.  It requires access to a lathe, so that might pose a problem to some readers.

Turn a pure lead slug about 1/2" to 5/8th long, to a diameter about .005 over what you expect the groove diameter to be.  My experience is that about .003 is easier to work with, but with some of the old original barrels it's more prudent to go a little over.  Run a oiled patch through the barrel.  Drive the slug into the barrel, (from the breech end, of course), with a piece of hardwood dowel.  This should be of an appropriate diameter to the barrel/chamber, and should be faced off square in your lathe.  You want to drive the slug in almost all the way, but stop at least 1/16" short of driving it all the way into the barrel. Now tap the slug back out using a cleaning rod with a flat tip.  Be sure to close the action on the rifle so the slug doesn't end up on the floor, and handle with care, as that pure lead is easily deformed. 

You should now have a mirror image of the barrel in front of the chamber.  Being pure lead the slug will really "slug" up, and out, and by leaving a little hanging out into the chamber, you will also have a good idea of the throat, if any.   

I've used round balls with good success with new barrels, but find the longer slug is better especially with old, original barrels, as it gives you a picture of more of the barrel.  This lets you see how much wear you have, and have to gas seal. 

Jerry