Oh hey it's my industry! We assume all of our clients 'lie' to us by default. We bake in a minimum of a 10% scrub-rate for our advertisers as a baseline, then it's a matter of seeing how much our numbers diverge when we check in with them.

For our publishers I had to write a script that checks for fraudulent traffic (based on a handful of heuristics/stats) and will ping our pub-managers if X-flags are raised for a given source.

The industry is definitely filled with garbage (I mean, it's marketing, anyone that participates in it is a plague), we've had companies just ghost us with a 100k outstanding bill, others that tried to DDoS us after we shut them down for fraud. Tracking your own numbers, and confirming with the wallet-holders on the client side regularly to make sure books match up is an absolute necessity.