Have you noticed that if you put your cell phone up to the ear that you don't use, it doesn't sound right. That's because your mind has trained your ear to use the right filters to compensate for the sound that your cell phone puts out.

It doesn't take that long but that is what is happening. If you are an audio engineer, you need to tune your ears.

Tools:

There are tools to help you do that. One of those tools is a good set of headphones. the reason you use headphones is because it takes the room out of the equation.

The next thing you need is something that will give you the tones or frequencies of interest. For live sound your ears need to recognize ⅓ octave frequency bands. So a tone generator or a CD with those frequencies on it is a must. Once your ears can recognize these frequencies it will give you the ability to find offending frequencies a lot easier then using an EQUALIZER you can take them out if need be. This is essential when you are dialing in monitors.

The next tool would be the ability to dissect a Mix or understand song structure.

Take a song and listen to it and try to map its structure, i.e. chorus, verse 1, chorus, verse 2, bridge, lead solo, verse 3, chorus, so on

Next find the instruments played in each segment.

Then map where they are placed  in the mix (extreme left or extreme right, or does it move back and forth) map it in clock degrees. 12:00 being center  7:00 extreme left and 5:00 extreme right this is the part of the mix that bring movement.

Level - what loudness is everything at in the mix. Is the snare drum as loud as the lead vocal? It helps if you use the Lead Vocal as a reference. Map everything in relation to the lead vocal. Pay attention to what happens when the lead vocal drops out. Does another instrument take over?

Texture - or EQ  how does that instrument sound. Is it natural sounding or does it sound beyond what the natural instrument is capable of sounding like. Is it deep or thin sounding? This is important because this will help determine genre. i.e. most songs use the same instruments (drums, guitar, bass, vocal) so what is the difference between a country song and a blues song in the way it songs and how we instantly recognize these things

Next on each instrument try to determine if there is an effect (reverb, delay, chorus,) if you don't know what these are you need to find out. this part is important because this determines the depth. Something that has lots of reverb sounds farther away. That is how we determine distance. The more reverb it has we perceive it as being farther away. For example background vocals should be at the same level as the lead vocal but should have more reverb on them to make them seem farther back than the lead vocal.

Last - What is the identifying sound of a Country, Hard Rock, Blues, or Jazz song. Once you can put your finger on these things you will be an excellent audio engineer.