You have to look for unnatural trading. Someone legitimately looking to sell shares wants to do so at the HIGHEST possible price. Manipulators actually look to sell lower, but as little shares as possible, because the goal is to move the market, not to make the best possible trade. Just one example. Another obvious example is when they trap the pps in a tight range or at a general level. It is extremely unlikely to occur naturally. A statistical improbability, and in defiance of normal human behavior.
I watch Level 2 when I can and sometimes it does get hard to follow. TD's phone app even comes with a smaller Level 2...It does sometimes help out in determining trading decisions.