What tests can a person perform to find out if biofilm in a hydroponic system is beneficial?

Biofilms usually contain a number of different microbial species, some of which may be pathogenic (but usually many which are also beneficial or benign in a healthy system). There are no basic grower tests to tell if the biofilm is harmful that I am aware of.

However, if the plants in the system are suffering from a root rot pathogen such as Pythium, then the biofilm is likely to be harboring this and will act as a source of reinfection even if the diseased plants are removed.

There may be some laboratories which could test a sample of a biofilm see if certain plant pathogens could be cultured, but this does not necessarily indicate that active disease is present or that the biofilm is harmful.

For more details on biofilms in hydroponics, refer to Is Biofilm in a Hydroponic System Bad?