Module globals
    Public Form1IsOpen As Boolean
    Public Form2IsOpen As Boolean
    Public Form3IsOpen As Boolean
    Public Form4IsOpen As Boolean
End Module

 globals.Form1IsOpen = True

 Private Sub Form1_Closing(ByVal sender As Object, ByVal e As _
   System.ComponentModel.CancelEventArgs) Handles MyBase.Closing
        globals.Form1IsOpen = False
    End Sub

if globals.Form1IsOpen = True then
msgbox("Form ist offen")
end if