Usually by trades. Everything gets traded nowadays, but that's only been going on for the last five or ten years. For most of the history of the medium, only stuff that sold well got collected, so you can generally tell how popular a series was by how much of it is available now. Other clues are spinoffs and guest appearances. Multiple titles under the same umbrella means the brand is selling well (JLA in the 90s, Avengers now). Guest appearances in "exposure" books, like Showcase or Brave and the Bold, usually means "not selling well, and needs promotion." Guest appearances by Big Names also usually mean a title's not selling well - think of all the cameos in Blue Beetle, or the way Batman's name is always huge on the cover of anything he guests in, because he's there to bring eyeballs.
And then of course there's run length. HUNTRESS didn't even make 20 issues.