What to listen for before mastering...
TIME-Make sure the elements of the song are in time with one another. The song will sound sloppy and from a listeners perspective will sound just bad no matter how good you get the tracks sounding by themselves. If a track was recorded in time with a click track/metronome we want to make sure that each track is in time with the metronome and also with each other. If something is out of time it should be rerecorded or corrected (if possible). It is possible to correct bad timing of an instrument without rerecording using certain tools inside your daw. But if possible always try to rerecord the audio, this is because some tools used in your daw will add a muffled or distorted sound after it has corrected the timing. If the song wasn't recorded to a click track again use your ears and ask yourself does this sound sloppy in any way, would this sound good if played over the radio? If its bad it should be rerecorded. Remember the master is only as good as the mix and the mix is only as good as the recording. Timing is huge in the overall sound of the mix. The engineer may say "well it sounds perfect" but the listener will say "I'm confused because that guitar sounds wrong and why did the drummer do that". Get the timing right before moving on with anything else.
CORRECTIONS-We want to listen for audio clipping, popping, hissing, crackling, and background noise such as air conditioning fans, paper tussling from the vocalist, dogs barking or whatever. We must get those noises or distortion of the audio out. we want nothing but the instrument to be heard. It's easier to do it at this stage than in the later stages.
TONE-When listening to the song ask yourself, is there too much bass, are the cymbals to loud, are the guitars hurting my ears after awhile, are the Vocals muddy, and so on. We want to find out what tracks are fighting each other for the spotlight. Almost all musical instruments will have a broad range of frequencies from 20 hz to 20,000hz meaning and acoustic guitar can play usually anything from 100hz and up audibly but in a mix we don't really need that 100 hz as they are in-audable and the bass will take those frequencies. So in short the acoustic guitar will fight with the bass guitar in that frequency range. Now is this audible in a full mix, not really but it's happening and needs to be fixed. This same thing can happen in any frequency range between any instrument.
EQING-When eqing a track, start with anything that sounds out of place, such as guitars that begin to hurt your ears after a period of time. The 2khz range is a very ear popping sound so to speak. Cutting this frequency range(2khz-3khz) (not completely ) will greatly improve the listening ability of your mix. Another frequency to cut is the 500hz range which can be called the mud frequency. All This is called eq cutting. It is better to cut rather than boost. Instead of boosting for example the low end on a bass track, try cutting the mud . Doing so will bring out the bass that is already there while at the same time cutting some noise. When you boost a frequency you not only boost that frequency but also the noise in that frequency range. Boosting and cutting frequencies should be used appropriately . Always try cutting first before boosting a frequency.