How to reliably measure working memory capacity? The capacity of human working memory is subject to individual differences. The number of items or chunks a person can retain in their memory may, have profound effects on a variety of other cognitive processes, such as problem solving. 

I have come across a couple of ways to measure working memory such as the delayed match-to-sample task, Dual N-back task or the Wechsler Memory Scale. For the latter I have been unable to find out whether it offers an estimate of the number of items which can be held in working memory. This, however, is precisely what I am interested in. I know that the capacity measured using verbal stimuli is affected by a variety of features of the material.

Therefore, I'm wondering what are the most commonly used methods to measure an healthy individual's working memory capacity. Also, is there a reliable measure that can be easily used in online research? I am also interested in measures suitable for laboratory research, though. Finally, are abstract visual material and digits more "pure" measures than verbal ones?

I am especially interested in standardized procedures with known reliability measuring working memory and giving an estimate of the number of chunks an individual can retain.
First of all, the existence of a working memory is a theory, not a proven fact. The working memory is a concept of the information processing theory that likens the human brain to a computer. Other theories don't necessarily have an equivalent for this concept.

Second, there are different theories regarding how the working memory is made up: one single store (e.g. Atkinson &amp; Shiffrin) or several systems (e.g. Baddeley &amp; Hitch). Depending on what you believe the working memory is, you might have to measure it (or its parts) in different ways.

Traditionally the capacity of the working memory is measured by asking subjects to remember a list of items and then counting how many they can repeat. This is a standard subtest in many intelligence tests, e.g. the WAIS. The average (!) capacity for remembering letters or numbers has been shown to be 7 +/- 2 . Different experiments find different numbers, but they all range between 3 and 9 (as far as I know). Regular tests stop at nine items, and it is very rare that someone can regularly hold more items in his short term memory after hearing them once only (telephone numbers are stored in long term memory after being learned).

Measuring the capacity for non-linguistic visual, non-verbal auditory, olfactory, tactile etc. stimuli is extremely difficult, because as yet scientists don't even begin to understand how these stimuli are percieved, processed and represented in the brain. The only comprehensible and meaningful measurements are made of the memory of linguistic items like letters or words or visual representations of objects (like a page with a number of drawn objects).