There are several ways to identify a potentially toxic thermos flask. Here are a couple of them:
Examine the interior of the flask. If it presents seams or welded cuts, then it is most likely a low grade flask produced with potentially dangerous material. Real steel thermos have closed tubular shapes without any joints.
Use a magnet to test the flask’s magnetic properties. If the magnet sticks, then the material is most likely not aluminium and may present potential hazards. High grade steels will usually not attract a magnet,but even this test may not be accurate, since some high quality steels can have a content of other metals that are magnetized.
Overall, there is no one simple factor to ensure that you are buying a high quality thermos flask, but rather a combination of factors.