I bought some Gluten at a health food store.
The store itself has a Belz hechsher.
The person who sold it to me is not religious. He told me that all the grains (including the gluten) sold in the store are ground in the Jerusalem mill and are under the supervision of the Bedatz. He said I can phone the Bedatz to confirm. Do I need to phone or can I rely on his word?
You can rely on his word. This is because if he will be caught lying, he’ll get into trouble with the owner of the store. Because it is something that can be easily verified (and he can really be caught lying), one can rely on the assumption that he is telling the truth.