To be competent an organisation or individual must have:
sufficient knowledge of the tasks to be undertaken and the risks involved
the experience and ability to carry out their duties in relation to the project, to recognise their limitations and take appropriate action to prevent harm to those carrying out construction work, or those affected by the work
Competence develops over time. Individuals develop their competence through a mix of initial training, on-the-job learning, instruction, assessment and formal qualification. In the early stages of training and experience, individuals should be closely supervised. As competence develops, the need for direct supervision should be reduced.
If you are engaging a person or organisation to carry out construction work for you, then you need to make a reasonable judgement of their competence based on evidence. The evidence will usually be supplied to you by the person or organisation quoting or bidding for the work.
There are many industry card schemes which can help in judging competence. However, the possession of a card by an individual is only one indication of competence. You are expected to make efforts to establish what qualifications and experience the cardholder has.