Just look at the gunk on this rod.  Again, here, the biofilm is easily seen and is obviously not a good thing.  How can you detect a biofilm if it is unseen?  In general, if you run your fingers over a surface of any kind and it feels slimy, chances are that it is coated with a biofilm.  The touch test is not failsafe, however.  A surface can look and feel clean and still be infected by biofilm.