system
A chat on subreddit /r/wallstreetbets
human
www.finviz.com has relative volumes next to the 3 indices. NASDAQ is trading at 1x normal volume while DOW/SP500 are trading aroud 0.5-0.6. That's what I use to quickly get a feel for overall market volume.

gpt
Relative Volume = Current Volume / 3-month Average Volume 
  
 So the 3-month average volume is the baseline. FYI - for FinVis.