Most states require that you supply references vouching for your work history and capabilities. In many states, some or all of these references must be licensed professional engineers. For example, here in Oklahoma, three of the five references must be licensed P.E.'s. If you work in a state that has an industrial or manufacturing exemption (discussed in a previous post) then you may have a problem finding references that are licensed. Or, you may work in a company that is out-of-compliance regarding the requirement to have a P.E. in direct supervision of non-licensed engineers.
How do you obtain a license if you don't have the required number of licensed references? First, contact your state board. They may have a special procedure to deal with this situation. Beyond that, you will need to see if your company is willing to provide the needed P.E. references by one of the options mentioned in the next section. If they are not willing, you should be looking for another job.
Out-of-compliance companies or companies with the industrial exemption
Some industrial companies may operate in states (such as my home state) that still require that engineering work be supervised by licensed engineers. If they aren't adequately audited by the state board, then they may become out-of-compliance with the regulations. Eventually, their employees may not have the needed references to become P.E.'s.
Options for companies with insufficient P.E.'s for either proper supervision or for references
This option could take years to accomplish because they would probably only be hired on an attrition basis.
The consultant option will only work if the consultant has frequent contact with the employees. For example, my previous employer hired a professor to come to R&D on about a quarterly basis to review the active projects. The employees would discuss their work and possibly make presentations of experimental plans and results. The professor would make suggestions, including providing literature citations for appropriate work. That professor, who had a P.E. license, would make an appropriate P.E. reference. It would be appropriate to alert the consultant ahead of time that he/she will be serving as a reference for designated engineering interns.
This option is for large companies that may be out-of-compliance in a portion of the company, such as R&D, but have P.E.'s elsewhere in the company. The P.E. that serves as the reference is obligated to be "knowledgeable" of the employee's work. One way to accomplish that is to include the task of serving as P.E. reference for X, Y and Z employees on the P.E.'s job description or performance review and then including he/she on all correspondence and reports by X. Y and Z employees. In addition, some scheduled meetings with the employees such as mentioned in Option 2 would be useful.
Some combination of the three options can be used to allow employees to apply for a license. It would be prudent for the company to discuss their plan with their state's engineering board to be certain that it will meet their requirement for P.E. references. Eventually, the company will have enough P.E.'s to serve as supervisors and/or references.
If you have a M.S. or PhD
The professor that supervised your project(s) in graduate school may be used as a reference because the work occurred after obtaining your B.S. Undergraduate professors most likely may not qualify since they can only comment on your course work or undergraduate projects, not on engineering work after graduation.
The fine print
The above suggestions are based on a discussion I had with the OK State Board of Prof. Eng. and Surveyors. However, you should consult with your state board if you don't have the number of P.E. references required.