DMA cheats are not detected. What happened is thousands of cheaters all bought firmware from the same guy, and Riot was able to determine via stats that this group of people with the same obscure "network card" had outlier stats, and they banned them all. DMA is by definition not detectable, but human idiocy is.
Not sure about KVM-like hardware cheats, specifically. You could obviously use an AI to simulate mouse movements, but I don't think that's particularly common.