How to tell why assets are being included in the built player

As recommended in Reducing File Size, I'm looking at the "Player size statistics" section of Editor.log to see what's being included in my .unity3d file. There are a few assets listed that I don't want included (some textures from the Standard Assets folder, for example). I assume I must be referencing them somewhere, but I've looked and can't determine where.

Is there an easy way to tell why an asset is being included, in order to track down the source and eliminate it (or leave it as is, if it turns out it really is needed)?

Not very easily. You can try building only a subset of all your scenes, and see if it gets pulled in then. If you only have one scene, kill half your game objects, and see if it still gets included. devidide & conquer.