We tend to hear our mixes miles different from other mixes even when they are extremely similar if not near identical. Its a skill I am just now beginning to obtain. Listening to your mixes objectively is hard as fuck to do.
+1000. This is one of the most frustrating things about mixing imo, and one of the reasons why it's good to get someone else to master your stuff if you have the budget for it.
It's also good to have a buddy or two who aren't AEs, but have decent listening skills, so they can give you feedback from a normal listener's perspective.
Another thing you can do is listen to the mix while doing something to keep yout mind off it, like playing a game or so. That way you don't overscrutinize things, but will notice right away if something is really flawed, cause it draws the attention back to the mix.
And the graph BLUElightCory linked is very good to be aware of. Guitars almost always have some nasty spikes around 4khz that can become harsh oh so fast when turning up volume, so keep an eye on that area. Most mixes that sound great to me have surprisingly dark guitars in general.
On the other side of the spectrum, it's good to set bass-levels on somewhat higher volumelevels (when you mix on whispervolume), since it can be deceivingly soft on lower levels, making you crank that 60hz knob way too much.
