How can tacit knowledge be measured? Question 1: If tacit knowledge is that which can't be written as declarative facts or communicated easily, then how can scientists measure how much of it someone has in a lab?

Question 2: Is it true that tacit knowledge can never be declarative or written down?

Example: Knowledge like "never tell your immediate boss in this organization what you feel about TPS reports, or you won't be able to get your complaint past upper management" seems like tacit knowledge. But yet it's written down as a declarative fact.
Short answer
Because tacit knowledge is defined as knowledge we cannot describe, researchers apparently adopt a strategy of just interviewing and listening for signs of tacit knowledge, as far as I understand it.  

Background
Tacit knowledge can be curtly defined as ‘that which we know but cannot tell’.

For example, a person may be very aware of being able to ride a bicycle and able to describe how they learned to do it. However, the person may not be able to describe critical aspects of biking, such as swift, reflex-driven adjustments due to imbalance, and subjects like the steadying effect of the gyroscopic motion of the wheels, may not be consciously realized either.

Still tacit knowledge can be made explicit by either making the knower to learn to tell, or by the researcher telling and seeking respondent. In other words, one has to facilitate the ‘telling’ of the untold. Both methods require the researcher to construct an account. 

Eraut (2000) claims that tacit knowledge can be examined through regular interactions, encouraging persons to freely describe what they know. This can be facilitated by an informal relationship, where one can freely interact with the person about their knowledge.

Reference
- Eraut, Brit J Edu Psychol (2000); 70: 113–36