As fat as ice observed on fan blades doing a walk around, it is somewhat subjective as I recall seeing photos on a PPRuNe thread that depicted ice on the back of the fan blade that is hard to visually observe...
I am surprised with the word subjective. I am recently retired, but I taught the deicing operators to always check the fan blades on every aircraft they were called to, and deice anything they found with a hot air blower. There was no subjective. If the pilots called for deicing, the deicing crew came out and deiced the fan blades if there was anything to deice. We had a contract with deicing that they would inspect al aircraft if asked, and not charge us if they were OK.