What is the prime sign that a country is transitioning from a developing to a developed economy? Developed Economy = low human capital flight, good infrastructure, low corruption, high per-capita income, dominated by hi-tech industry, high expenditure in R&amp;D, high-quality higher education, high-quality health care, and so on.

Developing Economy = high human capital flight, bad infrastructure, high corruption, mediocre per-capita income, dominated by agriculture or service sector, not enough expenditure in R&amp;D, low quality higher education, low life expectancy, and so on.

What is the first sign that a country is transitioning from a developing to a developed economy? 

If we want to point to 'only one sign', what would that be?

And, Why?
I don't know what the 'first sign that a country is transitioning from a developing to a developed economy', but I would look at the rates of output growth and study its acceleration. 

In a layman's words, a transition from low to high income regime requires convergence to a high income teady state. This is only possible if the growth rate of aggregate output, underpinned by improvements in all aspects of the economy, has been consistently higher than that observed in high income (which you call developed) economies. Transition is complete (or the country achieves a 'developed' status) when the economy's growth rate reaches its long run steady state rate (typically low (&lt;2% p.a.) positive growth rate).