The usage is extremely quick, and follows easy-to-spot patterns. We noticed a spike in bounce rate.
They never come from Google, and the bad programmed ones just crawl several pages at a time, faster than a user could do.
Then there's the crazy spikes in visits from specific countries, pretty much scraping the entire content. Often from pools of IPs. In some cases had 30% unexplained (meaning: it wasn't viral or a marketing campaign) random sustained increases in traffic.
There's also the fact they don't interact with the complicated widgets, so zero XHR requests other than analytics pings.
They also don't cause spikes in Google Analytics, so I assume it's blocked, but they show up in logs and in the internal analytics.
It's not enough to DDOS the website at all, but it's a lot of noise in statistics that we gotta learn to filter.