This is a tough question for a lot of employers and has far-reaching impacts for both the FMLA and the ADA. In some ways an employer does need to act as a bit of a mind reader as an employer’s obligation to provide reasonable accommodation applies to any known physical or mental limitations. This does not mean that an applicant or employee must always inform you of a disability, and often an employee is not going to directly request FMLA leave or state that they need an accommodation. If a health condition or disability is obvious, then it may be considered that the employer “knows” of the disability even if the applicant never mentions it.
If it is clear an employee is having difficulties with their work or attendance, it is time to begin the interactive process to determine if the employee has a disability and an accommodation is needed.