See the Difference

The most obvious way to notice an unmastered track is how flat the file looks in any audio editing software.

In the first screenshot mastering has not been applied. Notice how flat and lifeless the unmastered waveform looks.
View of the waveform of the Massaad + Garland song Atmosphere before mastering

After mastering you can see how much more definition the song has. The waveform in the screenshot below reflects this enhanced energy. You can see the difference!
View of the waveform of the Massaad + Garland song Atmosphere after the digital mastering