If you have doubts, you can certainly call in an unbiased third party to inspect the discovered problem. Most contractors, upon discovering a problem, will immediately show you what they have found and explain what the problem means, what additional work will be necessary, what additional materials might be required for the repair, and what the additional cost will be. If this does not happen and you don’t have a high level of trust in the contractor you hire, then you should ask more questions and consider a “second opinion.