Good question and it's changing.
The generally used and accepted measure/indicator is the Human Development Index (HDI). This is a number that ranks countries according to a combination of 3 main factors
- GDP (Gross Domestic Product) = money a country has/ national income
- Life expectancy = how long people are expected to live, when they are born.
However people argue that other factors should be included - for example FREEDOM levels, happiness, pollution...
Personally, I think these statistics help to direct donor money to countries that need it but also that everything is relative. I would rather be happy than rich ;) I think my Quality of Life or Standard of Living would be higher if I was happier and threw all my money away :)