While there is no “real” test to evaluate and rate the “totality” of the mind’s capabilities. IQ tests seem to be a valid and reliable indicator of the mind’s ability to comprehend and synthesis and apply information/skill to tasks within a “general” knowledge base.
IQ tests ARE culturally and socio-economically biased because a certain set of knowledge is assumed to have been acquired in order to be able to take the test. If that foundation is not there, it is difficult to assess how proficient the mind is capable of functioning.
Also, being that tests are stressful, people are being timed, the state of mind of the test taker, usually means people score lower than what they really possess in terms of IQ.
As you know, there is a good argument that self-esteem, perseverance, etc., are greater indicators in terms of predicting success and the ability of the mind to function on an optimum level.