How do you handle it?

I have run into adopted children before and I struggle every single time with whether or not I should flush out that line.  

Some answer themselves - for example the SSN entry identifies the birth parents which means the family knows - usually these kids were raised by an extended family member, but knew.

Then there are the folks who were adopted, names were changed and lived their entire lives with the adopted names - knowing or not the identity of their birth parents.

I  recently filled out a line as the family lore is that the child was adopted after his parents were killed - killed or died, I can't find the truth of it either way or the name of his birth family.

How do you handle these ones - do you flush them out or just add the person in the first instance with a notation of adoption? 

I'd love to hear how you handle these!