how to check type of ajax response

Hello, When my post message is successfully saved I am returning simple message via:

flash[:notice] = "OK"

and when there are some errors due to validation I return the message in similar way:

flash[:notice] = form.errors.to_xml()

then the message is sent via ajax:

respond_to do |format|       format.html { redirect_to :back }       format.js

How can I check on the client side which message was sent before displaying it? I am using jQuery. Thanks, bogumbiker