The real stuff doesn’t exist might be the best response.
Like any product unless the origins of the product, processing and finishing are all quality controlled and traceable through accredited suppliers there is no way of knowing for sure.
As @njwch2 asks or points out there is no clear position on how the products are described and marketed in Australia. Thanks @njwch2 for pointing this out.
A search of the internet left me with the impression that at least 99% of what is sold as made from bamboo fabric is simply synthetic (plastic fibre) made from bamboo, instead of petroleum products or more traditionally chemically dissolved trees? Please excuse the avoidance of chemistry and technical jargon.
P.S. (if you don’t have time to read the original source article for this topic, there are two very different products)
There are two ways to turn bamboo to into a fabric.
Mechanically by crushing bamboo, and over much time allowing natural enzymes it break it down. The fibres are combed out and spun into yarn. Similar to making linen, which we also love to wear close to our intimate parts? It’s also labour intensive and expensive. How close is the genuine bamboo product to linen which is a tough and durable product?
Chemically where the bamboo is dissolved and treated to produce cellulose. Not such a nice process environmentally. Think wood pulp and paper production on steroids. This is used to produce a rayon like yarn. It’s debatable whether the yarn produced from bamboo by this process is significantly different or better than any other similar synthetic when used to manufacture fabrics? Physical it may not differ at all, but in marketing it opens a whole new avenue for misleading consumers.
I could suggest plastics when they were first produced could truthfully lay claim to being 100% natural sustainable products. They were derived from trees, although as a slight difference bamboo rayon comes from a grass.