If is was me, I'll ask the current parent about the truth.
If the case it that I'm an adopted, I try to find out more about my
birth parents' details.
Then, I'll find my brith parent and reunite together. I know this
will hurt the current parent, but I have a suggestion.
My birth parents, will be the real
parent..
I'll make my current parents as god ma and pa.
Treat both side nicely.
In this case, when the adopted one reunite with the biological
parent, the current parent will feel very very sad.. 
Have BBQ to get to know more for both sides~
When having reunion dinner, invite your god parent too
v