There are different ways of determining this:
If you have the TCGA barcode, this is by far the easiest way. Look at the 'Sample" field
If you don't happen to have the TCGA barcode, then it's most likely a UUID, Case ID, or just a file-name that may have some ID in it's name. In these situations, you can search for these manually at the GDC Data Portal in the search box and then follow links in order to see if it's tumour or normal.
For example, if I have:
0b0e0b62-b823-4fdb-b37b-4a2731e648a7, this relates to primary tumour
3ba5d6ec-dcce-49bb-82e5-85d3903a2aa1.htseq.counts.gz, this relates to UUID
c247b168-3b4b-40ae-8e1a-32dda1b34397and is a normal sample
There are other automated ways of doing this but the ones that I tried appeared to be outdated when I recently used them (open to being corrected if wrong, though).