A foreign employee's wage is very important in the context of H-1B petitions, the labor certification-based green card process, and often Conrad-based J-1 waiver applications. In these contexts, the employer must demonstrate that it will pay the foreign employee the greater of (1) the actual wage rate (that is, the wage rate paid by the employer to all other individuals with similar experience and qualifications for the specific employment in question) or (2) the prevailing wage.
The prevailing wage is determined by reliable wage data for the foreign employee's occupation in the area of intended employment (that is, the area within normal commuting distance of the address of intended employment). Typically, this data comes from the Department of Labor, Bureau of Labor Statistics (BLS) under the Occupational Employment Statistics (OES) Program. This wage data is updated every year and is made available on the Foreign Labor Certification Data Center On-Line Wage Library at www.flcdatacenter.com/.
However, there are gaps in OES data for certain years and/or areas of intended employment. In those cases, there are often default wages provided by the BLS. Alternatively, an employer can file a prevailing wage request (which is required for the labor certification anyway) or possibly use its own reliable wage survey.
For your reference, there is much information on prevailing wage policy in 20 C.F.R. 655.731(a) (a provision of the Code of Federal Regulations), 20 C.F.R. 655.10, and the Employment and Training Administration Prevailing Wage Determination Policy Guidance (revised November 2009).