To me, the quickest way to assess the relative value of a bike in a given lineup is it has stamped (cheap) or forged (higher quality) dropouts. Nearly every major maker had a range of quality in their product line. At some point, maybe just below the middle of the range, the stamped dropouts end and the forged ones begin. Everything from there up is usually pretty good quality. Frame materials that are less sophisticated include "1020" and "High Tensile" (or "Hi Ten"). "4130" is okay but still not the best. When you see decals indicating the frame material and it says who made it (Columbus, Reynolds, etc.), you're getting into better quality stuff.

As far as wheels go, I agree that aluminum alloy is something to look for. They are a dull silver; steel wheels have a shiny chrome appearance. Stay away from the chrome. If the wheel is out of true side to side, as long as it isn't too bad, you can probably true it. If there are any discontinuties in the cross section of the rim from hitting a curb, or the rim has a "hop" (out of true in the radial or up-and-down direction), stay away. A hop often indicates the rim is bent and you can't get rid of it just by playing with the spoke tension.

One wildcard in all this is if you find an older English bike (Raleigh, Humber, Phillips, Hercules, etc.), if it looks at all salvageable, it's worth a look. They are throwbacks- heavy as heck. They often have Sturmey Archer AW 3-speed hubs with a year and month date code (like 65 12 for December, 1965). These are classics and if you can get one cheap enough, you won't lose money. Riding a properly tuned English Roadster like that is a whole different experience in cycling; it's like cruising in an old Cadillac. Very posh.