You ask for a scale to assess or measure the dimensions of word of mouth, such as credibility and sensibility. If I got it well, you are asking to what extent we can trust on another’s words, namely on his/her promises and commitments.
I think that Kohlberg's moral interview [See, Colby, A. & Kohlberg, L. (1987). The measurement of moral judgment. Two Volumes. Cambridge: Cambridge University Press] is a good, deep, albeit difficult, tool to assess to what extent we can trust on what individuals (e.g., politicians) say, namely on their promises and commitments, for example, to being sincere, trustworthy, and the like.
Think, for example, of the following question: “Should we keep our promises? Why/ why not?” According to Kohlberg’s theory of three levels (pre-conventional, conventional, and post-conventional) and six stages (1, 2, 3, 4, 5, and 6) of moral reasoning and development, individuals’ responses to this question or questions similar to this one and their justification for them are a way of seeing to what extent we can trust on others’ words, namely on their promises.
Consider the following response: “We should keep our promises because if we don’t do that we are liars, and can be censured and even punished”. This would be a stage-1 oriented response. It is likely that stage-1 oriented individuals are not sincere when they make a promise and, hence, we cannot trust on their words, “promises” and “commitments”.
Consider now the following response: “We should keep our promises because if we don’t do that, others will not keep the promises that they make us either”. This would be a stage-2 oriented response. Like stage-1 oriented individuals, it is likely that stage-2 oriented individuals are not sincere when making a promise and, hence, we cannot trust on their words, “promises”, “commitments”, and the like.
In Kohlberg’s theory, Stages 1 and 2 are pre-conventional stages in the sense that individuals at these stages are guided in their acting and thinking by individualistic and egocentric motives or reasons. Therefore, they do not see themselves as members of a society governed by social rules (e.g., traffic rules), moral norms (not to steal, not to break promises), and so forth.
Note now the following response: “We should keep our promises because if we don’t do that others are going to form a bad image of us, and we are not, say, decent and “nice” guys. This would be a stage-3 oriented response. Stage-3 oriented individuals tend to be more sincere and trustworthy than their stage 1 or stage 2 counterparts. Therefore, we can trust on their words, “promises”, “commitments”, and the like.
Note now the following response: “We should keep our promises because we should abide by the social rules, and moral norms of democratic societies. If this were not the case, there would be no social progress. This would be a stage-4 oriented response. Like Stage-3 oriented individuals, Stage- 4 oriented individual are more sincere and trustworthy than their stage-1 or stage 2 counterparts. Therefore, we can relatively trust on their words, “promises”, “commitments”, and the like.
In Kohlberg’s theory, Stages 3 and 4 are conventional stages in the sense that individuals at these stages see themselves as members of a society governed by social rules and moral norms (see above) and, hence, as individuals who have duties and rights.
Imagine now the following answer: “We should keep our promises. A promise is, say, a free social contract, and social life and societies would be impossible and even collapse if free social contracts were broken at any time. This would be a stage-5 or stage 6- oriented response. I say stage 5- or stage 6-orienetd response because the late Kohlberg gave up his stage 6 as an empirical and structural reality, even though he had maintained it as a moral ideal and, say, the telos of one’s moral reasoning and development. Individuals at these stages are highly trustworthy people and likely to act in accordance with what they say and promise, just to give an example.
In Kohlberg’s theory, Stages 5 and 6 are post-conventional stages in the sense that individuals at these stages think that moral norms (not to lie, for example) are relative and only make sense when they are guaranteeing prescriptive, generalizable, and universalizable moral or ethical principles, such as the principle of justice (i.e., not to treat others unfairly), benevolence (i.e., not to turn away from someone in need), social utility (e.g., to bring about the greatest good for the greatest number of people), or eudamonia (i.e., to bring about the self-actualization and self-fulfillment of every individual).
In a nutshell, there is amassing evidence that shows that the more one’s is advanced in moral/ethical terms, the more one’s is a reliable and trustworthy individual. This is one of the reasons why I believe that Kohlberg’s moral interview, which allows us to attribute a give level and moral stage to a certain individual, is a semi-structured tool to assess the extent to which individuals’ words are credible, sincere, and trustworthy.
I hope that I have got your question and that this helps.