Softness is a spectrum, and it varies by sport and market within the same book: a book razor-sharp on NFL sides can be soft on tennis or player props. The catch is well known - soft books defend themselves by limiting winning accounts, so the practical edge is bounded by how much volume they'll take from you.
Softness is measurable from the feed: compare each book's price to the devigged anchor at the same timestamp, and watch who trails after a steam move on the movers view. The +EV scanner automates exactly this comparison across the whole market.