How can we evaluate DBSCAN parameters?
OPTICS gets rid of $\varepsilon$, you might want to have a look at it. Especially the reachability plot is a way to visualize what good choices of $\varepsilon$ in DBSCAN might be.

Wikipedia (article) illustrates it pretty well. The image on the top left shows the data points, the image on the bottom left is the reachability plot:



The $y$-axis are different values for $\varepsilon$, the valleys are the clusters. Each "bar" is for a single point, where the height of the bar is the minimal distance to the already printed points.