- Alvin can time travel
- Both are a source.
- They both have a Dimeritium amulet. Aldersburg's is familiar, but older.
- They both are plagued by visions of the same thing.
- Aldersburg regurgitates the things you told Alvin. "You always said..."
- Aldersburg claims he owed you the very first time you meet him.
- You never see Aldersburg until after Alvin has disappeared.
- Alvin plays a game of "kill the elf" with his friends. He gets to play the Grand Master. And he always wins.
Some things are less obvious. Such as Aldursburg's opinions changing based upon dialogue. If you encouraged Alvin to become a Witcher, you find out Aldersburg was creating the mutants to be new perfected Witchers.
Mostly I think it is quite clearly signposted once the player accepts two things. 1) Alvin can time travel. 2) Blonde children aren't always blonde when they're older. These are the two things people miss the most. However, once Aldersburg's amulet is so prominently displayed at the end, it's an obvious tell in case they missed all the other clues.