Some things differ if you are looking at a new or a used frame, but here are some suggestions:
First, when you say you are looking for a steel frame, what you probably mean is a ChroMoly Steel alloy frame. This would be a frame made from tubes where chromium and molybdenum were added as the steel was refined. Frequently you will see a sticker on the tube with a number like 531, 631, 725 or 853 which is used to describe the particular blend.
Second, spotting a high or low quality frame has become more difficult over the years as machining and welding techniques have filtered down. With older frames you definitely want to look at the quality of the welds and brazing, as well as the paint. Welds may be smooth, or have a regular pattern. Watch out for welds that look rough, 'globby' or irregular. It used to be that the use of lugs was a hallmark of lower quality frames, but today you see them more often on high end boutique or custom touring frames.
You may also want to feel the bottom sides of the tubes for a "seam". On older low quality frames you frequently find rolled tubes, where the steel was a flat sheet that was then rolled and welded length-wise.
Finally, the durability of a steel frame is largely a function of the paint quality and maintenance. Water and rust are the primary enemies of a steel frame. If buying used you will want to closely inspect the area around the bottom bracket, dropouts, and the whole underside of the bike for chips, cracks and visible rust. For long life, as well as the low environmental cost, steel v. aluminum is probably a wash. In the US and most the developed countries both aluminum and steel recycling is around 50% and the production costs and impacts are pretty similar. And while corrosion may be an issue, an aluminum frame won't rust.