First thing to check is the condition fo your radiator, meaning is it clean. Shine a light through from the back and look at it from the front to see if the spaces between the fins are clean. If not clean then use a garden hose and clean it out.
Next pull the front rack and hood off so you can get to the radiator cap, open it up and check for fluid. Even though the overflow resivoir has fluid in it that does not mean there is enough fluid in the radiator.
While you have it open you can also start it up and look for water flow, it should be circulating pretty good. If no flow it could be one of two things, either the pump is not working or the thermostat is not opening. The thermostat is very easy to get to behind the side panel on the righthand side of the Grizz. You can pull this out and check it in a pan of water on the stove, unfortunately I can't remember what temperature it is supposed to open up at. My guess would be in the 125 degree range but that is only a guess. You can check it without the thermostat to see that water is circulating but I would not recommend riding it without a thermostat. The small engine could actually overheat more without a thermostat in it when you are riding allot. It will take longer to overheat but once it does get hot it will not cool down because the water is circulating to fast for the small radiator to cool it.
Last spring I had to replace a thermostat in my Grizz. It took awhile to heat up but once it got hot it would not cool down without a stop. In low range going slow it heated up quickly but if trail riding in high it took quite awhile to heat up.