I am in the process of revamping our reference checking procedures in the HR dept. Do we need consent from the prospective construction executive and what kind of questions are illegal?

When requesting references it is recommended that written consent be obtained from the construction executive job seeker. Candidates should sign a release directed to their current or former construction employer relieving the construction employer of any liability in providing reference information.
However, reference checking should be conducted regardless of whether or not a written consent is obtained. You can include an except and permission statement in your employment application procedures also.
The discrimination laws that apply to interviewing also apply to reference checking. In order to avoid claims of discrimination, decisions based on references should be made based on objective facts. Do not probe into marital status, age, disabilities, race, religion, or national origin.