As said above, you want to calibrate your playback environment first. Than, in my experience, with dialogue you want to be hitting equal to about -27 DB LEQ(a) up the center channel, usually measured on a stage with a Dolby LM100. That usually results in your dialogue average meter sitting around -16 dB to -12dB with peaks hitting around -6dB, maybe as hot a -3dB on a huge scream (this also depends upon how much compression you use on your dialogue, some prefer more than others). This method has seemed to produce desirable results which translate well. When I have leveled out my dialogue edits in the past with this dialnorm target even with the less-accurate Phasescope plugin and compensating for how it measures it's levels, I've heard the sessions thrown up onto the stage and the dialogue plays back at the right energy level and a good translation, granted the mixer will be further refining and adjusting it. Aside from that, as mentioned on here as well, with FX and MX it's all by ear/taste usually, since dialogue levels are the anchor to which you sculpt around. If there's any level 'rules' for FX or MX, I just make sure they top out at -1dB max, but I set a brickwall at -3dB so that only once a blue moon if something is just too loud and it pushes past the brick wall it will still be free of clipping. So about 99% of the time those things will top out at about -3dB and maybe 1% of the time they're reach up to -1dB.
This is all with features in mind though, as I understand TV works a little different, where I've seen it that dialogue is hard brickwalled at -10dB and the overall summed mix levels were hard brickwalled at -6dB and the LEQ(a) target can be somewhere arouound -24 or even a little hotter. I'm sure some TV mixers on here could chime in on that as I'm not very knowledgeable about that area.
My opinion is that dialogue is the 'black/white' anchor you want to establish level-wise, and you'll probably find that everything else for FX and MX intuitively fall into place once you've established that. Listneing to movie samples will definitely help give you a sense of the balance, but at the end of the day it's usually only the dialogue which has a hard and fast 'rule' as far as meter levels go.
As far as watching examples, as Matt said, I do recommend an exercise of caution because many times a DVD/BLuRay/iTunes is using a re-mixed version of the theatrical print, usually with a tighter dynamic range, and sometimes smothered with compression or even as much as 6dB hotter of a reference level (because a standard living room has an ambient 40 to 50 dB level whereas as theater is 20dB or so). Unfortunately it's not a well-known practice nor is it specified on the packaging, but it does happen and even then, there's non standardization in that some films are released to disc with the theatrical as-is (often quieter sounding), other's are remixed for DVD but keep the same reference level, and others are severely crunched and boosted during the remix.
I know one example I like to always go back to is MI:3 - unfortunately the compression on the DVD printmaster is palpable during the opening mission, to my ears almost like nails on a chalkboard. So I guess what I'm getting at is that Matt's ideas is a good one, although I'd recommend you more or less try to find a clear area of dialogue in the film first and level out your playback level otherwise you may very likely be comparing apples to oranges by having listened to the DVD too low or too hot. Equal loudness can affect how you judge the mix balance, as sounds become more impactful and punchy when they're louder but dull and rounded when they're lower.