Provided that User OnLine tracking is enabled in host settings, the following (untested) should work for you. Keep in mind however that the user on line status may not always be accurate as a user may have closed their browser or navigated away from your site without first logging out.

Public Function IsOnLine (ByVal PortalID As Integer, ByVal UserID As Integer)
       '  Obtain UserInfo object for UserID and PortalID
       Dim usr As DotNetNuke.Entities.Users.UserInfo = DotNetNuke.Entities.Users.UserController.GetUserByID (PortalID, UserID)
       ' Check OnLine Status
       If usr IsNothing Then
                Return False
       Else
                 Return New DotNetNuke.Entities.Users.UserOnlineController().IsUserOnline(usr)
       End If
End Function

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Bill, WESNet Designs
Team Lead - DotNetNuke Gallery Module Project (Not Actively Being Developed)
Extensions Forge Projects . . .
Current: UserExport, ContentDeJour, ePrayer, DNN NewsTicker, By Invitation
Coming Soon: FRBO-For Rent By Owner