you’re looking for any change in the cannabinoid profile or total cannabinoids.
If the total mg THC (weigh of input in g x potency in mg/g) in the system changes, you’ve degraded some of it (too hot). if it remains the same, you’ve simply not achieved purification (too fast). if your lab says you’v gained some, you’ve learned why smart folks do it themselves. 
If you see other cannabinoids (CBN, CBC*, delta8 THC) appearing, you also know you’re over cooking things.
*If they tell you you gained CBC, they’re wrong. Find a new lab.
THC Imposters (Isomers)