They can use a technique which involves an infrared survey of sorts where they could look for evidence of aliens who have used such energy sources.  They are assuming that there would be detectable changes in the environments and galaxies that would be very different from the usual things they would expect to see.  In theory this makes sense and sounds like a reasonable way to search.

Also the comments on this video thread are very spirited as one might expect.  Feel free to comment with your theories and if this is a reasonable way to search, or if other methods could be used.  Or, perhaps even in our own solar system such as on the planet Mars or Pluto, there is evidence of organisms or past civilizations.