The person who taught me: Kengo Yamada (owner of STYLE202)
Kengo Yamada, the owner of STYLE202, has spent up to 150 days a year in the US buying secondhand clothes. When purchasing, he must quickly sort through thousands of T-shirts, first sorting them by appearance and feel to determine the era.
"I pay attention to the shape and fabric. For example, before the 1980s, many clothes had short hems, but conversely, from the 1990s onwards, many clothes had long hems and a larger silhouette. The fabric materials also vary; in the 1980s, many were made of polyester blends, so they felt flat, while in the 1990s, there was an increase in thick 100% cotton fabrics, so they were white and fuzzy. From the 2000s onwards, the materials remained the same but the fabrics became thinner, so they looked lighter."
After making a rough selection like this, he then looks closely at the construction of the sleeves and neck, the text written on the tag, etc. Many of the points to pay attention to are things that Yamada learned from his predecessors when he first started working at a second-hand clothing store. He gave us a brief lecture on some of them.
First, determine the age by the neck construction
This classic 80s piece is made from 50% cotton and 50% polyester, and the body is by American brand devknit. 
Jack Daniel's printed T-shirt. Most of them are black, but green is rare. 
The print features Mr. Natural, a character created by underground cartoonist Robert Crumb.
Pakistani Maid x American Band = 70s
Tags are a valuable source of information. We tested them with skate T-shirts.
This is a piece from Santa Cruz, founded in 1973. The tag is still used on current products. The tag and the thread that fastens it are made of a polyester blend, giving it a sparkling shine. This type of construction is common since the 2000s. As proof of this, the back of the tag has "scskate.com" printed on it. 
Hook Up is an American brand that was launched in 1994. Its printed T-shirts were created by a passionate anime fan. They are characterized by two tags (one after the "S"). This was a desperate measure created to cover the numerous quality labels, and was widely adopted from the mid-1990s to the early 2000s. 
This T-shirt features the Shadow Man, one of Stüssy's iconic designs. The so-called "black tag" is proof that the item dates back to the 1980s. The "MADE IN U.S.A." inscription also provides a clue as to its age. Later, from the mid-1990s to the early 2000s, production moved to Latin America.
Guess whether it's an original or not by checking the sleeves, hem, and copyright.
It's a fact that the older something is, the simpler its construction is, and this is true not just for T-shirts. This is most clearly reflected in the stitching on the sleeves and hems. Roughly speaking, before the 1990s, single stitching was common, but since then, double stitching has become more common. Naturally, double stitching is more durable. Advances in sewing machine technology have made this specification the norm.
When you actually compare them, the 1983 version has single sleeves and a hem, while the 1997 version has double sleeves. Meanwhile, the 1989 version has a mixed design with single sleeves and a double hem. This shows the transitional period. If the date of the stitching matches with the year of the printed copyright, it increases the authenticity of the item.
sleeve 
hem 
Copyright
1983. 
1989. 
1997.
Regarding the handling of deadstock T-shirts
Deadstock items are available in the same unused condition as when they were made, but of course they are in a different condition than new items. Items that have been stored in a warehouse for years often have dusty fabric that has deteriorated. The store checks them, but in extreme cases they become more like paper than fabric, and some items will tear apart when washed in the washing machine.
It is said that black-dyed deadstock often suffers from rips and tears, possibly due to the dye. After purchasing, first soak it in water to remove dust, then wash it by hand. When drying, do not hang it on a hanger, but lay it flat, carefully. If you do this, you can wear it as if it were new from then on.
