Presumably the agency responsible for letting the contract, the agency responsible for procurement--which is to say, identifying potential sources of supply--would have a database of the various suppliers and would have their capabilities noted in terms of equipment, personnel, past work that had been done, and other indicators that would indicate their capability to do work of various types and complexity. When it came to qualifying people to submit tenders, the qualified people would be selected on that basis.
Again, before the request for proposals or the call for tenders went out I would recommend that there be a certain due diligence done to make sure you weren't going to be buying a bag of snakes.