If you look at the links on the site, you'll see that there is a download of the MS Office Automation help files, in these is the info you are looking for :
You can use the following function to determine if an instance of Microsoft Excel is running.
Function IsExcelRunning() As Boolean
Dim xlApp As Excel.Application
On Error Resume Next
Set xlApp = GetObject(, "Excel.Application"

IsExcelRunning = (Err.Number = 0)
Set xlApp = Nothing
Err.Clear
End Function
And then, your code can determine whether it needs to create a new instance or not…
Sub ExcelInstance()
Dim xlApp As Excel.Application
Dim ExcelRunning As Boolean
ExcelRunning = IsExcelRunning()
If ExcelRunning Then
Set xlApp = GetObject(, "Excel.Application"

Else
Set xlApp = CreateObject("Excel.Application"

End If
'Other automation code here...
If Not ExcelRunning Then xlApp.Quit
Set xlApp = Nothing
End Sub