In the middle of the very top of iTunes, between the play/volume controls on the left and the search bar on the right, there’s a panel that normally shows the currently-playing song name and position, or device sync info, or CD burning info.
Sometimes it doesn’t show me the currently-playing song name and position even though I’m playing a song and not doing anything else. There’s no little triangle button to switch between the things it should be showing, like I’ve sometimes seen.
To fix, close the iTunes window and re-open it by goning up to the Window menu and "unchecking"/selecting the (helpfully-named) iTunes menu option. The song you’re playing will continue to play, but the iTunes window goes away. Then pull the Window menu down again and "check"/re-select the iTunes menu option. The iTunes window comes back, and now it’s showing the name/position/album of the still-playing song.