How Can I Determine Object Distribution in a Multiple-File Filegroup?
You should assume, for all intents and purposes, that parts of every object in a given filegroup will be distributed across all of the files in that filegroup. You might be able to work up some script that will prove one or two smaller tables don't have any data on file 5 or file 8, but for what purpose? I say it's easier to just write a script that will rebuild all of the tables in that filegroup.