A quick-and-dirty way to find the peak is to select/highlight the part of the waveform you’re interested in, and then Effect → Amplify.
Amplify will default to the amount of gain needed to hit 0dB. In other words if amplify defaults to 4dB, the current peak is -4dB.
Then of course, cancel the effect instead of applying the gain change.
(If both waveforms are have 0dB peaks, they are both clipped and reading the peaks won’t tell you which is “louder”.)