This is far from foolproof,
but kinda creepy when it works.

NJs tend to indicate the future in front of them, the present right where they are standing, and the past right behind them.

NPs tend to indicate the future off to the right, the past off to the left, and the present a few feet in front of them.

When questioned, NPs generally say that they observe time flow past them, whereas NJs generally say they are right in the middle of the flow.

Here's another view of the same matter.