In short, a mix review is a free service I offer to all of my mastering clients (and it's something you can do too).
If the project timeline allows it, I do a full pass through the entirety of your record. I listen in my mastering room with no distractions (except maybe a cup of coffee) and a pencil in hand. No screens, no email, just a time to listen.
I then write down any and all thoughts that come to mind about the music. Notes about arrangement, overall tonality, things that stick out, moments that impacted me, places where I wanted more. After the listen, I distill my notes into actionable items which you as the mixer or producer can use to make your record sound even better. Suggestions may look like:
Track 1 - "My Awesome New Jam"
- The sub bass seems a little lacking on the kick
- LOVE the body of the vocal; sits very nicely
- Could use a bit more room sound out of the drums
- Some sibilance issues with the background vocals on the choruses
- Your toms are hitting like Floyd Mayweather; killer job
Whether you choose to implement the suggestions or not is your call, but either way, we both win. Why, you ask?
We both win because we've successfully opened a line of communication. I may have noted that "Bass feels a little too warm overall". You're able to see my remarks and respond in one of two ways, either saying "Thanks for that insight, I'll address it" or "I LOVE a warm round bass, please don't touch it!" The problem's either addressed or I then learn something about your tastes and can use that insight to give you a better sounding master.
I believe mastering is a creative and collaborative process between the mastering engineer and artist. The more we're on the same page, the better your music will sound.
Have you tried simply listening to your music? Give it a go and see what you come up with.