what do You mean by "Destructive"? AFAIK, only one way to check real R/W speed (be it sequential or random) is to actively perform test, that of course lessen total write cycles on that media (like any write operation), but I wouldn't call it destructive. What is destructive, is - probably - flash killer or something like that (tool to discover total amount of write cycles by actually writing it until it dies).

It would be really great to have on-device tool for testing purposes

/Estel