Go and read the Shuk Machane Yehuda Kashrut Report on The Yeshiva World website.
Short Version:
If a store does not have a valid teudah (kashrut certificate), don’t assume that it is kosher.
If a store says that they are Mehadrin but they can’t prove it with a valid teudah, then they probably are not Mehadrin.
If a salesperson [...]