Here's a rule of thumb I go with; if a "new" kit suddenly arrives from a manufacturer that does not have it in its line and there was no buzz about the "new" kit in the months preceding, it's probably a rebox.
Tamiya recently released an M109A6 Paladin to compete with the new Kinetic and AFV kits, a little research shows that they slipped the Italeri kit in the box and added their own figures.
Upcoming new kits are probably the worst kept secret out there. If an all-new kit is coming out, it's been talked about and photographs of the sprues dissected in most of the online forums.
And if it's an all-new kit of a never seen before subject, then there are probably two different companies coming out with competing kits.