There are basically two ways I know to find bad fonts.

1. Encounter errors when trying to use the font.

2. Use a font editor that highlights errors in font metrics.

Encountering errors is usually a hit or miss situation. You may use a font with some bad characters and not have an error occur for weeks, months, or even years.

Using a font editor is expensive and time consuming. I use a program called Font Lab. http://www.fontlab.com/font-editor/fontlab-studio/