"telltale" means his eyes betrayed or revealed that he was a stoner. "slits" means his eyes were narrow or almost closed. "practiced stoner" in this context means that the man was a frequent user of marijuana. (stoner = user of marijuana)

Steven was correct about 'telltale' being that they tell us about him, and 'practised' being 'experienced'. But he doesn't appear to know what a stoner is. A 'stoner' is someone who regularly uses Marijuana. If you use a lot of Marijuana, your eyes become red and narrow.

"Telltale," means that you could identify him as a stoneworker. His eyes "told a tale," - told us about him. idiomatic. Slits,- closed up eyes. The stone worker screwed up his eyes to protect them from flying bits of stone. They were red because of all the very small bits of stone hitting them. "Practised,"(US English spelling) is another word for "experienced."

it does not mean stoneworker in this sentence (context) it means long-term frequent habitual drug user (marijuana)