Apply this logic to whatever language you are programming in.  I won't speak it's name. 🙂
package gov.gis.leo;
import java.io.IOException;
import javax.swing.JOptionPane;
import com.esri.arcgis.addins.desktop.Button;
import com.esri.arcgis.arcmapui.IMxDocument;
import com.esri.arcgis.carto.IActiveView;
import com.esri.arcgis.carto.IMap;
import com.esri.arcgis.carto.IPageLayout;
import com.esri.arcgis.framework.IApplication;
import com.esri.arcgis.interop.AutomationException;
public class Button1 extends Button {
    @Override
    public void init(IApplication arg0) throws IOException, AutomationException {
        super.init(arg0);
        this.app = arg0;
        
        // Initialize our map document
        mxDocument = (IMxDocument)app.getDocument();
    }
    /**
     * Called when the button is clicked.
     * 
     * @exception java.io.IOException if there are interop problems.
     * @exception com.esri.arcgis.interop.AutomationException if the component throws an ArcObjects exception.
     */
    @Override
    public void onClick() throws IOException, AutomationException {
        activeView = mxDocument.getActiveView();
        
        if(activeView instanceof IPageLayout){
            JOptionPane.showMessageDialog(null, "You are in Page Layout view");
        }
        
        if(activeView instanceof IMap){
            JOptionPane.showMessageDialog(null, "You are in data view");
        }
    }
    
    private IMxDocument mxDocument;
    private IApplication app;
    private IActiveView activeView;
}