That isn't true. How do you figure that makes sense? The easiest way to figure out if someone is using Black Hat methods is to look at their Backlinks and see where they are coming from. Also, view the page source of their page and look for any hidden text. Search their website name in Google and see if any odd results pop up or their name shows up on any blackhat websites.
Yep, it's not easy to tell unless they are doing something extreme yet obvious (i.e. that leaves a big footprint), like using senuke or something
This is not an easy task to know whether the end user is doing black hat especially when they are building back links from illegal sites. However if they are trying to tweak their website by putting hidden text or cloaking the website then you can easily find it out and inform to the search engines.