A while back I made the assertion that modern dance has the ability to be all encompassing. In that, it is the only dance form that can reinvent itself and incorporate all ethnic styles of dance. Modern dance is the only dance form flexible enough to do this. Mr. Sweeney disagreed with me and brought up the choreographer Akram Khan as an example, a choreographer trained in the Indian classical form called kathak but also, may I add, trained in modern dance in England. He has lived in England most of his life absorbing, all the varied arts and sounds available to him there. I had the pleasure to see Kahn's "Ma" recently. From what I saw although Mr. Kahn uses both modern and Indian styles of dance and his music is primarily traditional Indian the only reason he can produce works in this style is because: 1. his abilities as a choreogrpaher and 2. his strong training in modern dance. Modern elements are the primary component in his work. As he says himself, in an interview with Philip Szporer 'he hires modern dancers because of their flexibility and ability to pick up his way of moving', he says, "Indian classical dancers are fed information in a very codified way. Contemporary dancers are risk-oriented, and quicker as a result in creating material."

To make the assertion that "modern dance has the ability to be all encompassing" one must first define what is and is not modern dance.
(And for that matter, how it can encompass other things, as well as what it might encompass.)
Becuase modern/contemporary dance is know for its ability to "reinvent itself and incorporate all ethnic styles of dance" I have yet to hear an adequate description or definition.
But without one, how can somebody pick out "modern elements" from a contemporary Kathak performance?

There are many books and articles written giving definitions of modern dance. Anyone with some understanding of dance can identify a performance of dance for what it is, ethnic, modern, ballet or a combination of these.

The definition of contemporary dance is certainly merky water. In my experience it is difficult to derive a unifying description across culturally specific work. The theatrical constructs that would define contemporary Kathak are certainly different from those that would define contemporary hip-hop. However, most dance communities define for themselves what is contemporary and what is traditional, and while this is always in transition, it is something that can be articulated when contextualizing an individual performance.

I don't know if I would agree in all circumstances with Georgie's point that anyone with some undestanding of dance can identify the disparate dance styles represented in a piece. Many choreographers amalgamate dance styles in a way that make it difficult for audiences to keep track of the dance genres that generated the choroegraphy. For instance, traditional haitian dance and Dunham-based modern dance are siblings, and without knowing the context of who the choreographer is, one might have difficulty placing the work they are seeing. What I mean by this example is that the intent and background of the artists, the biography of the creators, is a useful tool in tracing the movments root (sometimes more so than mere movement analysis).

I would offer that even if one makes a wrong stylistic appropriation, it is always a good idea to be aware of how we categorize the work we see and try to develop a deeper understanding over time of how that matches the intent of the aritists (i.e. whether they feel they are pushing the envelope in some specific tradition, or letting their incfluences run free in their aesthetic).