Which layer of PCB is used to identify cut-out of individual boards within a panel?
I've always used the dimension layer for marking out anywhere there should not be PCB, whether that's the outer edge or a cutout. I believe it is traditionally used for that (with 100 mil wide lines). 

However, if you're marking out boards that should actually be separate pieces you should use the 'Vscore' layer. This notes the a score line, for snapping boards apart should be placed. If you're planning on getting back boards that are completely cut apart then you just order more than one in the case of a duplicate or separate orders in the case of different designs.

The best bet in the end is to just ask your board house. It's them you're trying to describe your design to with the layout, so just ask what dialect they use.