If you know when the Mixing is done? This question is like asking a chef if the soup is done. It 'one thing to know. We can determine that point in the recording of commercial as one in which the optimum Constrained reached. This is the point at which the artistic vision realized practically subject to budget constraints; you know that the soup is done by Online Mixing and Mastering.
Some folks will always be adding 'finishing touches.'
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Test the song on several completely different sets of speakers.
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A good indication you're done is when you can play the song on your computer, on your Boom Box, in your car, and on your home stereo . . . and STILL hear every part of the song.