82 gl500 overheating4

I took my bike into the shop abiut a week ago and had them replace the front transmission cover, flush the cooling system and a couple other minor things.. when i hopped on the bike and rode it home, it started to overheat, and did not before that. I've only had the bike for a month....don't think too many services were done on it before me...it has 22k on it and when the guys flushed the system they said the coolant that was in there was like slush. from the looks of it there is no leaks from the radiator so i don't know where to start. fan not running right? bad thermostat? headgasket issue? keep flushing the engine?