In season seven, episode five, Gilly stumbles across information about Rhaegar Targaryen's secret marriage to Lyanna Stark while she and Sam are reading at the Citadel, and although she isn't aware of the words' implications, Sam later connects the dots. He speaks with Bran about it during the season seven finale, and when Bran confirms that Jon Snow's real parents are Rhaegar and Lyanna, Sam mentions the couple's marriage, prompting Bran to visit the wedding.
Please, please let Sam's knowledge mean that he survives! He's one of the true MVPs of the series, and in a popular theory among fans, Game of Thrones is actually Sam's retelling of the story years later.