keep in mind the code requirement that the "contractor's inspector" needs to ensure that the welding being performed is in accordance with any of the wps's issued to the shop floor. as a reference, look at D1.1 clause 6.3...that entire clause was re-written in the 2006 edition and tightened up in 2008 and continues as such in 2010.
based on that code requirement, i tend to "grill" the fabricator's QC more than the welder on the wps. "to whom much is given, much will be expected" or something like that..
whether or not a welder fully understands the wps as it is written can greatly depend on their comprehension abilities and besides, honestly, some wps's are written so poorly that even the best trained person has difficulty comprehending it.
maybe what matters more is that the welders understand the purpose of the wps and how following it as closely as possible is a good thing. i like to illustrate it like a "recipe"...if the wps is well written every welder can "add" their own little "dash of spice" or whatever and the "dish" will still turn out ok; others will like it and ask for more!
when i interview welders i never expect them to know the wps verbatim..."do unto others as you would have them do unto you"...but i do hope that they can take me to where they keep the wps's and show me the one they are actually welding to and maybe, just maybe, they can find the essential variables that they are expected to work within. if they struggle with that, either they didn't pay attention during the training class, or there was little or no training.
on the other hand, i like to interview fitters as well because there is quite a bit of information on that wps that applies to them; joint fit-up, etc.
oh well, that's my 2cents
ziggy
I made welding manuals for each of my welders. As they pass the welding tests, they get a copy. I spend time teaching them to read and understand what all is in the manual and how to find everything they need to do their job. All WPSs, general welding notes and instructions are included. I provide a detailed index so they can reference things quickly and not feel too anxious when under pressure to provide answers to the auditor's questions. Our shop is audited annually by the AISC and many other 3rd party auditors throughout the course of a year. They get plenty of exercise thumbing through their manuals proving to the auditors that they know what they are doing. 
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