The packaging and what it says about the product
Short answer: A pack with a clear size, fibre content and care instructions in several languages is a sign of a proper supplier. It says nothing about the fabric, though.
What the packaging has to contain
Size, fibre composition in percent, care instructions and the maker's name and address. Those four are a legal requirement in the EU and have to follow the product.
What you can read from it
If the text is correctly translated and complete, the supplier has the paperwork in order. That makes a claim easier because you know who is responsible. See bamboo underwear for men.
What it does not tell you
A handsome box does not mean a better fabric. Judge the fabric with your hand and not the pack. See bamboo t-shirts for men and bamboo briefs for women.
Frequently asked questions
Should you keep the packaging
Keep the label with the fibre information until you are sure you are keeping the item.
Are plastic bags a bad sign
No. They protect against damp in transit and are standard.
What if information is missing
Then you can ask the seller for it. They have to be able to state it.