Here's the part that applies to using upcycled type fabrics.
If the garment has FTC care tags you already should have the fiber information, And here's what the FTC says about those labels:
Replacing another company’s label with your own40
An importer, distributor, or retailer may want to replace the original label on a textile product with a label showing its company or RN. This is perfectly legal as long as the new label lists the name or RN of the person or company making the change.
NOTE: If you remove a label containing required information, the label you substitute also must contain that required information. Otherwise, you’ve violated the Textile Act.
SPECIAL CAUTION TO RETAILERS:
Some retailers, such as bridal salons, remove labels with required information from the garments they offer for sale without replacing them. This is illegal under the Textile Act. If a retailer removes any label containing required information, it must substitute another label with its own name or RN and any other required information that appeared on the original label. In addition, anyone substituting a label must keep records, for three years, showing the information on the removed label and the company from which the product was received.
In the case if you don't have labeling, or partial content is known then you use one of the below methods of labeling.
Products containing unknown fibers30
If a textile product is made — in whole or in part — from scraps, clippings, rags, secondhand fibers or fabrics, or other textile waste materials of unknown and, for practical purposes, undeterminable fiber content, the disclosure may indicate that this is the case. For example:
Made of clippings of unknown fiber content
100% unknown fibers — rags
All undetermined fibers — textile by-products
100% miscellaneous pieces of undetermined fiber content
Secondhand materials — fiber content unknown
45% Rayon
30% Acetate
25% Unknown fiber content
75% Recycled Wool
25% Unknown Reclaimed Fibers
60% Cotton
40% Unknown fibers — scraps
If on the other hand, the fiber content is known or can be readily determined, the full content disclosure must be given.
But in the case of buying recycled clothing the above may apply more often.