There are several ways to visualize forces in the viewport. The easiest way is to turn on the Guide parameter on the POP Force. This will display the direction and magnitude of the force. In the following example, there is a directional force of 2 in the X axis.

If you increase the force to 10, the size of the guides will increase.

This is especially useful for visualizing noise.

You can also turn on Display Point Trails in the Display Options toolbar, which illustrates the direction the particles are moving, scaled by the timestep. The length of the guides show the actual time moved.

Additionally, you can create custom vector displays for forces in the 3D Display Options dialog by clicking the New vector display option button on the Markers tab. Some useful attributes for POPs are force and targetv (for POP Wind). To turn on visualization in the viewport, simply choose it from the Display custom attributes dropdown menu in the Display Options toolbar.