Thanks for putting this question to Google on our behalf.

Impression fraud though hard to spot does have its signifiers. Significant above average increases in impressions and falling click throughs can be a tell tail sign for those of us that monitor our accounts with a fine tooth comb.

Impression fraud only works with the instigators ads paused so as not to effect themselves. So keep close tabs on your competitors.

As a practice I think it’s only a problem for highly competitive, high profit markets currently, but as knowledge filters through the rank and file over time it’s likely to become more widespread.