George, if I just can use indicators to do this, I would try to identify the forces of supply and demand that drive the market. In this sense, it would be good check prices and volumes, instead of just prices and a long term timeframe.So, my answer is select a price one, like moving average, as pointed out by Malacarne, and a volume one to double check, like OBV or MFI.There are options to find just one that join prices and volumes, but maybe not so easy to work as two specialized ones.
See these articles :
The Indicators of the Micro, Middle and Main Trends
I tend to try to find long-term use weekly or monthly chart with MACD and stochastic.
They are the 2 most reliable for me, especially when it crosses up or down from level 0.