I was given a similar task a few months ago and my solution was along the lines of the suggestion of glen.greaves -

identify the records with mismatches by using

select table a

union all select table b

except

select table b

union all

select table a

I had a function which took two fields and the name of the field and returned a value if the fields didn't match

field by field of the records that didn't match I looked for the field(s) with mismatches

oh and the first step was to limit the rows in table a and table b to records that matched on key fields