h2testw.exe is the program you want to use to test flash memory capacity; you should do it will all flash memory, including branded products, because fakes can and do get into the supply chains.
It will read and write a unique file to EVERY empty cell on the device, then read it back; fake devices arent usually picked up by normal testing suites, as they test each cell in turn with the same data file, and the fake circuitry can fool them by writing the data to the real cells and claiming to have written it to the non existent cells.
It is slow though, so you will want to leave it running overnight to test a 1TB drive.