I presume you mean an Excel Workbook/Worksheet. As far as I know you have to open it first but you can do that easily with the following function
Private Function GetWorkSheetNames(ByVal ExcelFile As String) As String()
Dim names As String = ""
Dim xls As New Excel.Application
For Each sheet As Excel.Worksheet In xls.ActiveWorkbook.Worksheets
names = names & vbCr & sheet.Name