One of the surest ways to know that you are not getting into a trade too early is to wait. I know, it seems quite obvious, but in this case we are to wait for a very particular occasion – indicators crossover.
When you are using several momentum indicator, which is advised, and one of them is already displaying a sign of divergence with the graph all you need to do is to wait for another one to confirm what you already thinking.
Look at the picture. After the crossover and the divergence, the price went down. But those, not waiting for a crossover might have easily sold out too soon.