With digital audio clipping occurs at 0dBFS.
You'll see the two red rectangles above the meters light up if the audio clips.
To adjust the audio for just one track; you can set a keyboard shortcut for Audio Gain, and then it's just a question of Alt Clicking on a clip to highlight just one of the audio channels and then hitting your shortcut to turn it up or down.
To prove to yourself where clipping does occur, place a bars and tone clip on a sequence. (Get the bars and tone from File > New > ). It should replay at -12 dB. Apply a +12dB audio gain increase to the clip. Use the rubber band on the audio part of the clip, drop the audio level at the start of the clip to a low level, and after several seconds place a second node at maximum gain. You will than have a slowly increasing in level audio tone. Listen to it and when clipping occurs you should hear the tone turn from a pure tone to one including harmonics, making it sound "hard". The further the audio is driven past clipping, the greater the harmonics.
It should clip at 0dB. On a new system that I was using recently, I thought that the audio did not sound quite right. Its a BM Intensity card, HDMI output to a TV monitor, audio out of the TV to speakers. Indeed upon using the above procedure, audio was clipping at -12dB! It turns out the TV wont give clean analogue audio out. It also has an optical audio out which is clean right up to 0dB. So that's the fix in my case.
To answer in a more general way, I try to keep my audio averaging at around -6 dB. That allows some louder things such as gunshots, or sirens, or loud yelling to go higher, let's say just under 0.0dB. But most of the audio runs at around -6dB.
I came to that answer after getting annoyed that my audio was always a little louder or softer than professional videos I hade downloaded, or played on a DVD.