Sorry for my late reply (out to lunch)...
Remember that 8.2.3 also contains a note for guidance:
"When determining suitable methods, it is advisable that the organization consider the type and extent of monitoring or measurement appropriate to each of its processes in relation to their impact on the conformity to product requirements and on the effectiveness of the quality management system."
Would evidence of monthly spot checks prove monitoring is taking place?
Yes, as long as you have kept some type of record proving this happened.
So that begs the question: What is "monitoring"? How would that be different for a Document Control process than measuring.
While I don't see any definitions for these two words in ISO 9000, it is not really that hard to distinguish the two...just a simple query in dictionary.com yielded these results:
Monitor: "to watch closely for purposes of control, surveillance, etc.; keep track of; check continually.
Measure: to ascertain the extent; dimensions, quantity, capacity, etc., of, especially by comparison to a standard.
So let's apply this to document control:
Monitoring example: For this month, you have checked all the documents on the floor and on the server and determine that the versions all match up against the master list (here you have found an aspect of document control to be effective, because the document controller is doing a good job, at least for now - no action needed).
Measuring example: You decide to do the same thing as above, but now you want to display it as a percentage. So 100/100 of the documents on the server are of the correct version, or 100% effective, but only 95/100 of distributed hard copies are of the correct version, or 95% effective (here you have determined a measurement of effectiveness from which to act upon - we need to investigate the root cause of wrong versions on the floor so we can raise this %)
Depending on the type and extent of control you want to apply, as stated above, either way can be utilized. On the subject of document control, some people may not bother with quantities or values (and that's fine) while others may want to know this stuff because of their statistical obsessions and whatnot.
Numbers are hard to argue with, though. But common sense usually prevails and most people don't have to overkill things like document control.
Does this make sense?
Hope I've Helped...

Brian