Is there a way to positively identify a maker of a coin set? For example, if you have a hopping half set or a copper silver brass set, how would you know if it was made by Todd Lassen, Jaimie Schoolcraft, Roy Keuppers or any other custom maker? Do they mark them in any way?

Most of the custom gaffers add a hallmark, usually on the inside/underneath.
The mass ones (Tango, Johnson, etc.) don't usually have this, so I'm not aware of an easy way to tell.

Older Lassen stuff won't have a hallmark, and I don't think Jamie ever put a mark on anything.
You can tell a flipper is a Lassen by the way it's constructed - internal band and the flippy part is a single piece.