Hi chanpou,
For these types of questions you would be primarily reliant on the reputation and integrity of the manufacturer and the accuracy of the information they provide. Some latex layers will still have the stickers on them that identify which type of latex it is but this depends on where it was cut from the original mold. The 100% natural will have an “N” rating and the blended will have a specific number as an ILD rating.
At one point Literie Laurier was only selling blended Talalay (see post #12 here and the rest of the topic about the Costco topper they were supplying) but they may have changed this since then.
Phoenix