How does Trailwise decide two records are the same run?

It tells you, on the activity itself. Each activity says where every field came from and which records sit behind a merged row, and it says in words how the match was made: a single source, or the same sport, day, distance and climb within a tolerance. It also says how confident it is that the activity lines up with a planned session. If it got it wrong, you can match an activity to a planned session by hand, and undo that again.