Here's a trick I use in thrift stores to quickly sort through shoes on a shelf- look at the leather-if it has a slightly roughened surface like the skin of an orange it is cheap leather and a mediocre quality shoe. With a little practice you will know what I'm talking about.
If you see threads on the sole- a sewn sole- this indicates at least a minimally decent level of quality. Lack of visible threads does not necessarily mean poor quality, however.
Look inside the shoe-is the lining a fine, soft leather that has been sewn in-this indicates high quality
Learn to identify shell cordovan leather-there are threads discussing that here- shell is an expensive leather only used in high quality shoes
Square toed shoes are highly likely to be poor quality-(although some good makers have produced them, but you wouldn't want to buy those models anyway
Made in Italy, England, France, or USA is a possible indicator of quality, though no guarantee
Quality of a shoe is not correlated to its weight- high quality shoes may be either light or heavy, poor quality shoes may be either light or heavy as well.
Go to a good department or shoe store and examine and handle shoes of brands you know to be high quality, then go to a cheap shoe store such as Payless and examine those shoes.It's a real learning experience you will soon be able to pretty easily tell the good from the junk