There's nothing in the law that requires you to send someone on a Sunday evening at her convenience.

If so, what are the licensing requirements? 

To determine the quality of a property requires to evaluate all of its return as well as potential return.  

its very easy.. tehre are 35 states that do NOT require any license to do NON owner occupied loans.. very simple  talk to local attorney have them prep your note and mortgage and your good to go.. should not cost more that 1k or so for the docs..

From a transactional cost standpoint alone, the fees on the way in and on the way out require significant appreciation to break even.

However, keep in mind it may complicate financing, add costs, and require careful management to maintain liability protection.

It should be a requirement to be educated in that space prior to licensure.

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

If you operate as a separate business entity - S-corp, C-corp or a partnership - you may be required to have formal compensation for yourself: either a W-2 salary or so-called guaranteed payments.

And remember, lenders change their requirements over time, so what they will do today may not be what they will do next month.