There are several rules to follow when determining a stronger conjugate acid.
As nj_bartel said, resonance and inductive effects play their roles.
So does charge, bond strength and electro-negativity.
You could ask yourself what the weaker base is.
charge:neutral bases weaker than anionic
bond strength:decreases down a group, weaker bond is weaker conjugate acid
electro-negativity: increases across period (more EN=more stable=weaker base)
resonance: more resonance=more stable=weaker base
inductive effects: nearby EN atoms pull charge towards them and cause weaker base