Q: Can I ask a Employee that drives a tractor trailer for medical clearance , if I observe him looking medically unfit?

A: Under the ADA, a disability-related inquiry or medical examination of an employee may be "job-related and consistent with business necessity" when an employer "has a reasonable belief, based on objective evidence, that: (1) an employee's ability to perform essential job functions will be impaired by a medical condition; or (2) an employee will pose a direct threat due to a medical condition. If an employer has a reasonable belief that an employee's present ability to perform essential job functions will be impaired by a medical condition or that s/he will pose a direct threat due to a medical condition, the employer may make disability-related inquiries or require the employee to submit to a medical examination. Any inquiries or examination, however, must be limited in scope to what is needed to make an assessment of the employee's ability to work. Usually, inquiries or examinations related to the specific medical condition for which the employee took leave will be all that is warranted.

The situation you present is fact specific, and a full consult with an employment lawyer is recommended. Generally speaking – employees in safety sensitive positions (truck driving) risk failing DOT medical clearance requirements with limb numbness. If your employee reported limb numbness, requestioning medical clearance & making reasonably disability-related inquiry for the sake of ensuring compliance with DOT regulations would likely not offend the ADA.