Bidirection ≡ Ex-Nor ≡ iff ≡ Equivalence operator ≡ <=>
p<=>q means (p=>q )AND(q>p) which means (p'+q).(q'+p) which means p'q'+pq
Now the prepositional logic p'q'+pq would be true when either both p,q are false or both are true.so this make p,q logically equivalent iff p'q'+pq is true always.
like for example let p=a+b' and q= (a'b)' so as you can guess p is logically equivalent to q here
but you can prove it by using p<=>q if the outcome for all possible inputs is always true then p and q would be logically equivalent.