Er, it does, in the window at the bottom - your master is on 0, the other channels all show the usual dB reduction in the boxes.
In simplistic terms, red is bad, green is good, and the box also goes red if an over level event is detected - so you could adjust the master fader so that you are close to, but not exceeding 0dB - for the obvious distortion reasons. People nowadays do not do this - mainly because it means your music will be louder than somebody else - imagine going to iTunes, or youtube and having to turn down every one of your tracks, compared to others. Overall dynamic range is now really great, so there's no need to push things. If you bring in an audio CD - a commercially produced one, look at the waveform, it doesn't remotely peak at 0dB any longer. In cubase, the mix bus is the usual problem area when you have lots of loud tracks - hence why the common advice is NOT to normalise - if you start to have to bring down the master significantly, then you are running very hot, and even with floating point processing, there are limits. If you look at a few waveforms of commercial CDs in your genre of music, see where they peak and try to replicate it in yours - the -18dB figure seems a sensible one - but you can push it from there if you have to. In a busy mix, it's still usually possible to have the working area on each channel somewhere around the ¾ fader travel point, as you can go up and down from there quite simply. It's not an absolute, you need to use your ears to detect things not quite right. Output level is pretty flexible from the avoiding noise and distortion perspective, but -18 gives a rough conformity between recordings. Don't forget that not all tracks need to be the same average, either. So a loud piece followed by a quiet piece shouldn't be at the same peak level, it sounds wrong.
We produce lots of show tracks, and because the playback kit is very variable, we do record these hotter, just a bit, because with an audience, a quiet track may require a fader prod that can't be done - it's already up! -18dB might seem a bit low, but it does work, and is a kind of standard, just subject to a little genre +/-