http://www.wunderland.com/WTS/Andy/Games/TwinWin/index.html
If at any time the appropriately colored Tree or Nest shown on one of your cards appears somewhere on the board, then reveal your card and claim victory! (Pieces under your Tree or Nest don't matter, but no other pieces can be in-between.)
Still doesn't cover all the cases: Can a piece be on top of the winning tree or nest? You might say "That can't happen, because you'd have already won before that extra piece landed there." but that means you forgot that goals can be changed, and it's possible that when the structure was originally formed nobody had the appropriate goal card.
-Dale