To preview the rendered look of the strokes as you paint, click on the Draw as mesh option in the panel.
Alternatively, you can render the strokes as you paint them in the Paint Effects Tool scene painting view. The scene painting view is a snapshot of the scene view. You can zoom, tumble, track, and dolly this view, and the scene painting view will update accordingly. You can render all the Paint Effects strokes or render them individually (no other objects in the scene render).
Paint in the scene painting view when you need to see what the strokes look like, but you don’t want to do a final render (for example, when you are defining new brush presets). You cannot use the transform tools, or any modeling tools in this view. Although you can select objects in the scene painting view and transform them in the Attribute Editor or Channel Box, it is best to use the scene view for non-painting tasks.