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ClickGuard detects VPN, proxy and Tor traffic by checking each IP address against several services that track which addresses belong to public proxies, VPN providers and Tor exit nodes. When an ad click comes from one of them, ClickGuard flags it, because hiding a real location is a common sign of click fraud.
ClickGuard uses different services to retrieve information about an IP address, which includes knowledge that an IP address is used by a public proxy or VPN - or Tor browser.
VPN / proxy providers constantly add new IP address ranges to their IP portfolio, which is necessary for them in order to deliver the service for their users (e.g. overcome geographical limitations in consuming content).
Therefore proxy detection can never be 100% accurate and different services will vary in accuracy - usually in detecting newly "proxied" IP addresses as proxies. When it comes to quantifying the accuracy it quickly becomes an impossible process since proxy detection services do not provide such information (everybody claims to be the "most accurate" and kinda leave it at that).
See also: What is a proxy, VPN, or Tor.