Feel the radiator hoses to see if they're all hot. Radiator could be dirty and need to be washed out especially around the back side. Plus you can do a radiator pressure test. Must hold 10psi. If it leaks down could be because of another blown gasket,a warped head or cylinder causing the problem.

 

How did this story end head gasket? I know old feed just curious

 

idk how it ends but my 1998 sportsman 500 4x4 will blow into the overflow when I rev high read up on several things about it and one I think is with my case waterpump bearing (if bad or going out can let crankcase pressure though and blows out at full throttle