It is a good theoretical concept stressing on letting your winners run so one can squeeze more profits out of a position. You know how we human traders get excited by even looking at a small profit. I think its natural because profit isn't yours until the trade is closed, and loss is all yours...
There are so many ways to define a trend, but you can simply get an idea by visually inspecting the long-term chart of SPX or any security for that matter... Looking at moving averages, trend lines etc..