How to get only the non-matching entries from an outer join

This comes up all the time. I'm trying to add a foreign key that wasn't previously modelled, but there are records that don't have their foreign keys in place yet for whatever reason. Usually a parent was dropped and without the FK modelled it couldn't cascade leaving the orphaned records.

I end up finding these by doing an outer join to a temp table, and then iterating through the temp table looking for NULLs and reporting on them, or clearing out the "bad" records.

There ought to be a better way to do this, but for the life of me, I can't think of it. I'm worried this is one of those questions with an obvious answer, but it just hasn't popped into my mind.

Thanks in advance for any help!

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Matt