the fan will NOT come on until the motor oil hits a certain temp, if you wish, you can BUY a service manual and or find a FREE Download one and in it will tell you what exact temp is needed to make it come on and it will also tel you how to TEST the fan to make sure it works properly!\
NEW to YOU atv's, the BEST thing you can do is do a full set of service PM's
that means change all fluids, so you know there all good and fresh, motor oil.filter, diff's, make sure air filter is clean and oiled if the type!
you also didn;t mention how many miles or hours on motor, and this IMO matters at times when asking for help or questions on things, more we know, better advice/help we can get you!
Congrats to you on the new to you atv as well, 450's are good atv's! and welcome to the site!