Before the US was so identity heavy[1], it was not an uncommon phenomena for a family to raise a child as their own[2] and the child not be told. That crazy uncle or great aunt might find out and might not.

In more modern times, questions generally get asked as a result of some blood tests or organ compatibility tests.

1) birth certificate for everything, although I remember an old story that the birth certificate might have been written by a sympathetic official

2) sadly, some stigmas were pretty great