I think most big bike builders made some decent ATB and rigid mountain bikes in the 80s and early 90s. I personally own three, a Schwinn High Sierra, Mongoose ATB, and Cannondale SM800 (aluminum, I know) all from the 80s and all of which have been converted to drops. I think finding a good steel mtb is no different than it is when you're looking for a good road bike. Forged dropouts, decent tubing, alloy components as opposed to steel, etc.