The methods I am familiar with are the Severn gage and the ferrite scope.
You can find more info at:
http://www.severnengineering.com/
http://www.fischer-technology.com/products/mp30.html
Also, I believe the AWS A5.4 and/or A5.9 standards have a very good discussion in their appendix on ferrite measurement in welds.
Marty