How can I tell if a file is from a tumor or normal sample?

Selecting the files of interest in the GDC Portal and adding them to the cart will give access to the "Sample Sheet" file. This file contains many fields like "File Name", "File ID" (UUID), and "Sample Type" for each file. The "Sample Type" field will denote whether the sample is from tumor or normal tissue and the other fields can be used to locate the appropriate files.