The main indicators to look for will be a severe drop in the stock market and steepening of the yield curve.
The yield curve is quite flat, the 5yr/2yr is inverted but the rest is not. But a steepening of the curve would probably still be a strong indicator the economy is going to contract.
Stock market valuations are historically high and a severe fall in stocks will indicate recession. (mainly because societies shared social mood influences peoples decisions in the economy/financial markets and this is reflected in stock valuations). This measure of valueations has a -.89 correlation with subsequent returns)
Stock prices and recessions
Much of GDP growth has come from a reduction in unemployment from very high to very low levels. Productivity is not improving greatly so a change in the unemployment situation will easily tip the economy into contraction. Scroll down to the section on growth….
Unemployment is not increasing yet but its already extremely low.