Make sure you control which countries your ads can show in. 90% of my budget for high tech equipment was getting spent in Bangladesh. Pure fraud. Once you start clamping down a LOT, results can get better.
If the % is high, then there is a case to be made that most of the clicks are from abroad VPNing into the US.
There are companies out there that provide paid anonymous, reliable (not hacked) proxy servers that run on consumer internet connections though, and those would be very hard to detect as vpn / proxy connections.
Another way is through capabilities where you have a computer that identifies itself as a macOS device but has the TCP/IP stack of a windows device – that's a proxy ! I would recommend the source code of p0f or the book silence on the wire to learn more about the topic.
Bangladesh didn't make the cut. :-)