 The four most common types of ad fraud are pixel stuffing which is the placement of an ad in an invisible pixel that's not seen on a page, ad stacking where ad placements will be layered on top of each other, only one will be seen but an impression will be counted. Misrepresentation or misidentification of a domain that can be declared as a quality site but in reality it is a illegitimate site made to deceive the buyer. Ad injections where a publisher is not given permission to an application or browser to insert an ad onto their page. The bad actors and ad fraud are anywhere from an individual that is looking to set up shop and make a quick buck to large organizations that have sophisticated distribution models to create illegitimate traffic that pulls from marketers sites. One of the biggest ways for a brand to realize they've been exposed to a high amount of fraud is the actual performance of their campaign. When non-human traffic and fraudulent traffic makes up a major component of an online program the performance will not be there. A secondary way for a brand to understand if they've been exposed to ad fraud is that the metrics are too good. A hundred percent viewability or zero exposure to ad fraud as measured by any third party is not likely to be legitimate traffic.