Sometimes, especially when we've been working on a song or track for a long time, it can be hard to tell if a sound is too loud or too quiet in the mix. Or if an effect is too intense or too low in the settings. We've become deaf to the details.
There are two things you can do about this. Either, if you've been working on a song for lots of hours, there's no deadline looming and it's late, you can just go to bed. Seriously. Chances are that you'll only muck up the mix. Being tired can also mess with your sense of hearing. Tomorrow you'll most likely be able to listen with fresh ears.
Sometimes though, this just isn't an option.
So what if you've got to finish because you've got a deadline and you're running late?
The way our senses work, is that something that is continuously present is simply filtered out. This doesn't just go for our hearing, but also our other senses, like our sense of smell. Every second there is a lot of information our mind has to process. To keep us from going mad, anything that isn't important needs to be filtered out. Otherwise we'd be bombarded with countless pieces of information and our minds would be overloaded in no time.
While this filtering is clearly useful, it can cause issues in mixing. A way around this, is to work with extremes in mixing. Are you unsure if a particular track has the right volume in the mix? Take the fader all the way down and slowly bring it back up. Or bring it all the way to the front then slowly down again. This way, it's much easier to find the sweet spot.
The same goes for the amount of wetness of effects. If you're uncertain about the amount of reverb on something, take it all out so you can only hear the dry sound. Now dial the wetness or send back in.
This works, because with the extreme changes we suddenly process the sounds anew that had become too familiar to be properly registered. Don't believe me? Just try it.