this might help: it shows each table, and what filegroup a table belongs to. if you only have ONE datafile per filegroup, then the table is contained in that datafile.
if you have mulitple datafiles per file group, then as Scott Identified, the data is placed in a round robin fashion, so you it's distributed across all datafiles for the filegroup.
I think there is an information schema that basically ahs the same data, but here you go:
SELECT OBJECT_NAME( i."id" ) AS TableName ,
i."Name" AS IndexName ,
FILEGROUP_NAME( i.groupid ) AS FileGroupName
FROM sysindexes AS i
WHERE ( i.indid IN ( 0 , 1 ) Or i.indid < 255 ) And -- Tables & indexes only
OBJECTPROPERTY( i."id" , 'IsUserTable' ) = 1 And -- User tables only
OBJECTPROPERTY( i."id" , 'IsMSShipped' ) = 0 And -- No system tables
COALESCE( INDEXPROPERTY( i."id" , i."Name" , 'IsStatistics' ) , 0 ) = 0 And -- No Statistics / Auto-Create stats
COALESCE( INDEXPROPERTY( i."id" , i."Name" , 'IsHypothetical' ) , 0 ) = 0 -- No Hypothetical statistics
ORDER BY FileGroupName , TableName , IndexName
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