Are you in the USA? The law does not allow for requiring documentation. You can only ask two questions - 1) Is the dog a service animal required because of a disability? 2) what work or task has the dog been trained to perform?
That’s it.
You ABSOLUTELY can ask what tasks the animal performs. If the guest says it is an emotional support animal then by ADA definition it is NOT a service animal as defined by the ADA and outside of the scope of that law.