Rey herself, Daisy Ridley, apparently thought the answer was hiding in plain sight.
If she considers the answer to have been in that movie, then following that logic, whatever in that movie hints at her parentage is the answer.
-The obvious first thing that comes to mind is the quote: "That lightsaber was Luke's, and his father's before him and now it calls to you!" Immediate, strong implied connection.
-The second obvious thing is that all she wanted throughout the years was to see her family again, and she repeatedly had dreams of Luke's island during this time.
-A third not quite as obvious hint is that Kylo Ren, who trained under Luke, has a peculiarly strong reaction to and interest in Rey even before meeting her and discovering her power. That could indicate a connection from his past.
People have wondered if those are red herrings because they're too obvious, but if Daisy is saying that the movie holds the answer in plain sight, then that means she is related to Luke for sure.
Or, at least she was intended to be when that movie was made. Rian Johnson could have made the decision to go another direction of course. It's fiction and it's still in-progress.