as a general rule, looking at the periodic table can help:
N2 obviously has equal EN values, and shares perfectly equally (covalent)
SO2 has S and O which are both in the same column, one above the other, so they are relatively close in EN.
ClF3 has the same situation as above but are to the right (more EN)
LiF are hugely separated left to right, but are in the same row as each other.
K2O is also separated a lot left to right, but K is two whole rows below O, so therefore it has the greatest difference in EN.