Yesterday, i went to shop but all of the shirts of that shop was labelled with 100% cotton tag. But It don't seem to be truly 100% cotton if i touch them.
Simple technique to test natural fibers like cotton and silk against synthetic.
Find a thread of the cloth sticking out and using a lighter, burn the end. If it burns to ashes, its is natural. If on the other hand it 'melts', it is plastic (polyester, acrylic, etc).
It would probably be best to remove a thread and take it outside the store to a windless place ;)