I don't know if this is useful, but sometimes, I think it was like the day after, my villagers would rant about redd, and asked me if I wanted to buy their painting off them, I did once, and it was a legit painting. It happened with another villager too, but it was fake. I didn't care about too much about losing some bells for another try. x)
With villagers selling you art, you can't tell whether it's real or fake, but you may as well buy it since if it's real you can sell it for at least 100k on these forums.