is there a certain rule you can follow or no?
Exactly what type of withholding are you talking about? If we are talking about FIT only, there is a box the employee's W-4 that can be checked. If you are talking about some other type of tax, you are going to need to get a lot more specific. There are 100 or so FICA exceptions and each state has its own rules.
To add to DW, who cares? An employer is the conduit. If the employee gives you valid forms/instructions, it is not your obligation to question the accuracy. An employer has plenty enough rules to follow, as well as running their business instructing employees in the "art" of taxation.