Overflow tank keeps filling up with water

I noticed this yesterday. Since I recently got the car back on the road with a new radiator I didn't think much of it. I just poored some out and went out riding around for about 2 hours. When I got home I noticed the tank was completely full again and running over. This is a familiar problem, the old rotary did the same thing because of a blown seal between the rotor plates. I'm thinking it's a head gasket, but since magnum heads are notorius for cracking could it also be that? How woud I find out?