Here is a throwback from Maister, Green, and Galford from ~25 years ago: The Trust Equation

T = C + R + I / S

T= Trust; C= Credibility; R = Reliability; I = Intimacy; S= Self-orientation 

An example using a simple 1 to 10 rating (1= terrible, 5= average, 10= world class):

5 = 7 + 8 + 10 / 5 (in this example, the client has an average level of trust (5) with his/her service provider) 

Try it out on your relationships. Do your clients trust you?