If you want to know whether your file is indexed in Sharepoint Search Services Provider (SSP) or not then there is no way provided directly in the object model of SSP.
Since Sharepoint stores its content on SQL Server therfore, you can run the following TSQL query to find it:
Select * from dbo.MSSAnchorText where Link='<Your File Path>'
Note: Please note you need to replace <Your File Path> by the full path of the your intended file in the above query
Example
Select * from dbo.MSSAnchorText where Link='http://mossdev/Shared%20Documents/UserManual.pdf'
