With a shotgun bore micrometer.

http://www.brownells.com/aspx/ns/store/catsearch.aspx?c=1384&p=4810

You can use simple calipers but all you will be able to measure is the constriction at the crown of the barrel. That will not tell you the actual bore size. And just because you can measure the end constriction that will also not tell you exactly what choke your barrel may be because you don't know how large the bore was before the choke began constricting.