The idea is to put a drop(s) of oil, presumably ugly black oi that is highly visiblel, on the spot you want to evaluate.  Then go fly for enough time for the air flow to "streak" the oil in the direction of airflow.  It doesn't always work very well.  Gravity will affect the trajectory of the streak, and in the back of the plane the boundary layer is sufficiently thick that little streaking may occur - at least in a reasonably short time.  And then, of course, the time spent in the desired flight regime (say, cruise) must be enough longer than the other conditions (taxi, climb, etc) so that the streaking is accurate.  The method doesn't cost anything, so it's usually worth a try.  And then different kinds of oil will produce different results - I haven't experimented with this.

And I agree that a single dorsal fin would be better than the twin ventral fins.  Ventral fins are used when there is limited vertical clearance, like on a jet when a tail "rub" is possible.  They are also used when increased pitch stability, especially in high AOA condtions, is desired.  Neither is a factor in our planes.  I looked hard at building a dorsal fin, but finally gave up and just bought the ventral fin from Custer.  The thing that tipped me over the edge was the difficulty of providing a tail tie-down.  In retrospect I should have stuck with it.

Gary