What version of Access?
Access 2000 or newer?

If Not CurrentProject.AllForms("FormA").IsLoaded Then
    ' FormA is not open
   Do something here
Else
    ' FormA is open
    Do something else
End If

In Access 97, copy this function (from the Northwind.mdb sample
database) into a module:

Function IsLoaded(ByVal strFormName As String) As Integer
' Returns True if the specified form is open in Form view or
Datasheet view.

   Const conObjStateClosed = 0
   Const conDesignView = 0

   If SysCmd(acSysCmdGetObjectState, acForm, strFormName) <>
conObjStateClosed Then
       If Forms(strFormName).CurrentView <> conDesignView Then
           IsLoaded = True
       End If
   End If
End Function

Then code some event:
If Not IsLoaded("FormA") Then
    'FormA is not open
   Do this
Else
    'FormA is open
   Do that
End if

Why?  You've described "how" you are trying to do something (using two
forms, checking between them).  Unless you also provide some specifics of
"what" you are trying to do, you won't get the full benefit of having all
these folks here in the newsgroup.  It may be that there are alternate ways
to get done what you want to, if only we knew what that was...