Even after thorough research, a person’s parentage may remain uncertain. It is useful to record these tentative connections — the proof might come later — while making it clear they are unconfirmed.
Set the Certainty on the Families tab of the Current Person Other Detail dialog (click the ‘…‘ button next to the name fields). The options are "possible" and "probable".
When the certainty is set to something other than confirmed, the connection between the child and their parents is shown with a dashed line on charts, rather than a solid line: