How to reliably measure working memory capacity?
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).