What to do if an employee was intentionally miscategorized as an exempt employee so as to save their employer from paying them overtime?

The issue of whether someone is an exempt or non-exempt employee depends on the facts of their work relationship with the company; it has very little to do with what an employer labels a worker. The definition of "exempt" and "non-exempt" has much more to do with an employee's level of responsibility or their status as a professional.  

The U.S. Department of Labor specifically designates certain classes of workers as exempt, including executives, administrative personnel, outside salespeople, highly skilled computer-related employees and licensed professionals (doctors, lawyers, architects, engineers, certified public accountants, etc). In addition, managers who hire and fire employees and who spend less than half their time performing the same duties as their employees are typically also exempt employees. In general, the more responsibility and independence or discretion an employee has, the more likely the employee is to be considered exempt. Installers/laborers would not appear to qualify.

Employers who have violated any provisions of the Fair Labor Standards Act ("FLSA") are subject to civil penalties; this includes employers who have miscategorized employees as exempt in order to avoid paying them overtime. The employee can file a claim against the offending employer with the US Department of Labor (the wage and hour division in their state), their state's labor department and/or consult directly with an employment law attorney in their area.

In the meantime here is a site that you may find to be of help: http://www.dol.gov/whd/flsa/