Air signs: Very hard 

Air signs only trust their intuition, hence it is very difficult to trick them

Earth signs: Hard

Earth signs prefer actions over words, hence it is harder to trick them

Water signs: Slightly easier

Water signs are susceptible to believing flattery and compliments and may fall into other's traps more easily

Fire signs: No effort needed at all 

Well...  as long as you are willing to say it, they are willing to believe it