How does one find out the capability of an individual in his own field?

We can judge a person on 2 things:

The exposure the person got within his/her field - Means complexity of situation, responsibilities person got, issues person might have faced.

other thing is how person can handle situation and where the persosn stands:

- With guidance and instructions

- With minimal guidance

- By him/herself

- Expert in handling his/her situation 

I think if we can co-relate above both things - situation/exposure and personal capacity we can judge person's capabilities

Each individual requires certain competencies to execute any tasks in his area of expertness. There is a proficiency level needed for each level. On an assessment center competencies and proficiency level deviation can be assessed with the help of multiple assessment tools  

Each role in an organization have certain set of KRA's. These are measured by certain set parameters which are formulated based on business expectations. The capability of an individual can be judged by the performance in these goals over a period of time

Apart from these they can also be measured by Tests ( Technical & Psychometric) , Team performance , subject expertise, etc