Mixing / Production

I recently watched a YouTube video in which it was recommended that you match the input and output level of every plug-in in a mix session...

By stepping back and taking stock, our engineer manages to rescue his own mix of guitars, bass and hurdy-gurdy for an Alt/Folk band!

How can you tell that a particular recorded vocal or bass track includes resonances? I’d like to know how to identify where the resonances are visually.