Reign of Giants is being folded directly into the Don't Starve Together code base. That means that there will not be any way to check if RoG is present for DST: it will always be there. There is, however, a world gen preset that allows you to make an (almost) vanilla DS game in DST. I'm not sure off the top of my head if there's a dead-simple way to query if that's being used, but it should be pretty easy to cook up, if not.