It's true that the money tree seeds are more gold in color. When you have several special seeds in your inventory, click on the folded corner to see the whole stack and the money tree seeds will be more gold than the other seeds in the stack. I can't really tell when they're on the ground, though. Just always pick up any special seeds. The cemetary is always a good place to look, but surprisingly, there are special seeds literally all over town. If you can purchase the Collection Helper with happiness points, it will be well worth it and you'll be stunned at how many seeds are laying around everywhere!