Bounce to tracks... use your eyes... play through momentary peaks to see how high they get.
This is a good way to not only see peaks but to see areas of the song with lower peaks.
It helps to see average volume and levels by frequency too, but to do that in Sonar you really do need to play back while watching meters.
For people with partial hearing loss, average volume of left and right channels measured independently is also useful.
Depending on your instrument panning, you could have more left channel peaks than right, but more average volume in the right. Then, limiting those left channel peaks might increase the left/right average volume disparity as the volume of both are raised but the limiting largely impacts only left.
For average volume measures, I use the DigiCheck EBU meter. Any EBU would do.
If you want peak measures to be channel independent, bounce to mono tracks and pan left and right. Then solo them together for playback through the obvious peaks... Sonar will measure peaks on the channels independently.
Once you have an EBU meter, you can use the old solo frequency range trick with multiple buses and the Multiband compressors to measure the average amount of frequency for various ranges. I don't do this much, but you could. I usually just watch a spectral analyzer... sometimes channel specific.