public bool IsSelected { get; }

Public ReadOnly Property IsSelected As Boolean

using DevExpress.Pdf;
using DevExpress.Xpf.PdfViewer;
using System.Windows;
using System.Windows.Input;

namespace DetermineContentType {

    public partial class MainWindow : Window {

        public MainWindow() {
            InitializeComponent();
            viewer.OpenDocument("..\\..\\demo.pdf");
        }

        private void viewer_MouseMove(object sender, MouseEventArgs e) {

            PdfHitTestResult result = viewer.HitTest(e.GetPosition(viewer));
            string contentTypeText = result.IsSelected ? "Selected " : "Unselected ";

            switch (result.ContentType) {
                case PdfDocumentContentType.Text:
                contentTypeText = contentTypeText + "Text";
                break;
                case PdfDocumentContentType.Image:
                contentTypeText = contentTypeText + " Image";
                break;
                case PdfDocumentContentType.Annotation:
                contentTypeText = contentTypeText + "Annotation";
                break;
                default:
                contentTypeText = "The content is empty";
                break;
            }
            barButtonItem.Content = contentTypeText;
        }
    }
}

Imports DevExpress.Pdf
Imports DevExpress.Xpf.PdfViewer
Imports System.Windows
Imports System.Windows.Input

Namespace DetermineContentType

    Partial Public Class MainWindow
        Inherits Window

        Public Sub New()
            InitializeComponent()
            viewer.OpenDocument("..\..\demo.pdf")
        End Sub

        Private Sub viewer_MouseMove(ByVal sender As Object, ByVal e As MouseEventArgs)

            Dim result As PdfHitTestResult = viewer.HitTest(e.GetPosition(viewer))
            Dim contentTypeText As String = If(result.IsSelected, "Selected ", "Unselected ")

            Select Case result.ContentType
                Case PdfDocumentContentType.Text
                contentTypeText = contentTypeText & "Text"
                Case PdfDocumentContentType.Image
                contentTypeText = contentTypeText & " Image"
                Case PdfDocumentContentType.Annotation
                contentTypeText = contentTypeText & "Annotation"
                Case Else
                contentTypeText = "The content is empty"
            End Select
            barButtonItem.Content = contentTypeText
        End Sub
    End Class
End Namespace

<Window
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:dxb="http://schemas.devexpress.com/winfx/2008/xaml/bars"
        xmlns:dxr="http://schemas.devexpress.com/winfx/2008/xaml/ribbon"
        xmlns:local="clr-namespace:DetermineContentType"
        xmlns:dxpdf="http://schemas.devexpress.com/winfx/2008/xaml/pdf" x:Class="DetermineContentType.MainWindow"
        mc:Ignorable="d"
        Title="PDF Viewer" Height="350" Width="525">
    <Grid>
        <dxpdf:PdfViewerControl x:Name="viewer" MouseMove="viewer_MouseMove" >
            <dxpdf:PdfViewerControl.CommandProvider>
                <dxpdf:PdfCommandProvider>
                    <dxpdf:PdfCommandProvider.RibbonActions>
                        <dxr:InsertRibbonPageGroupAction PageName="{x:Static dxpdf:DefaultPdfBarManagerItemNames.MainRibbonPage}" Index="4">
                            <dxr:RibbonPageGroup Caption="Content Type">
                                <dxb:BarButtonItem x:Name="barButtonItem"/>
                            </dxr:RibbonPageGroup>
                        </dxr:InsertRibbonPageGroupAction>
                    </dxpdf:PdfCommandProvider.RibbonActions>
                </dxpdf:PdfCommandProvider>
            </dxpdf:PdfViewerControl.CommandProvider>
        </dxpdf:PdfViewerControl>
    </Grid>
</Window>

PdfHitTestResult result = viewer.HitTest(e.GetPosition(viewer));
string contentTypeText = result.IsSelected ? "Selected " : "Unselected ";