When running a song through the "Test" option in the beatmap editor, a running timer of the song should be visible on the game screen. Occasionally I'll want to do an entire run-through of a song, and perhaps about two and a half minutes into the song I'll think to myself, "these three beats are off". But in order to find those specific beats, the only thing I really have to go by is the colour and positioning, and perhaps relation by ear. Having a simple clock-timer would make fixing timing issues a bit easier, I think.