Look for high quality leather, good insoles, and lining. A high quality boots should be cut specific to your feet. In many cases expect to pay over $1K for a good quality custom boot. There are alternatives such as the $650 +/- custom boots from the good works company Escanaba Polo Mallet Co.
I would think in terms of quality of the boot relates to 1.) Design, 2.) Fit (Shoe Size), and 3.) Leather Type (Real Leather)
Here are a couple of strong articles about the quality of polo boots, an equipment list, and a buying guide to leather.
Why Polo Boots Are Considered One Of The Best