Ask them what kinda notice they need and how late they work.

Seems simple enough to me.  The key here is finding out who works late, and then how much notice you need to give them.

however, if you want a late night early morning visit, you should prearrange it.  If you are looking to do something last minute during those times, get "the reader" from your local chicago news stand or check the yellow pages in your area under "escorts"!  After Dark and Gentlemens Pages magazines might help as well!  Check your local adult book store for these publications!

Thanks for the suggestions...  

but you would need to be pre-verified since it is nearly impossible to check a gent out very late at night

Just make sure you are screened ahead of time since obviously trying to get references at a late hour can be difficult.