This article talks about asbestos being identified as carcinogenic in the 1920's, but the ancient Greeks knew not to buy slaves from the asbestos mines.
But I bet they didn't diagnose it as "cancer", just "will die soon"
Are you sure? The word cancer comes from Greek - Hippocrates named it Karkinos, which was translated into the latin cancer a few hundred years later. The name explicitly comes from the appearance of a cancerous tumour.