A particular species is collected and is described as "dark red-brown". How could we tell if this alga is a member of the Rhodophyta or of the Phaeophyta?

The Rhodophyta are red in color and Phaeophyta are green in color but Phaeophyta’s body may be filamentous or may show root like hold fast, stemlike stipe and leaf like blades. Many of them have air bladders which keep them afloat