You can check the names of teachers you are interested in out, to see if they have any background or have made a name for themselves playing the game. Established and long running schools are less likely to be scams, but it is worth checking on forums to see whether it is something people recommend or warn against. Beware of claims that are too good to be true, like if someone says they will make players millionaires overnight.
The other thing I would say is learn from several people. There are many different approaches to playing cards, depending on whether you play the casino, other players, the game you choose etc, so you need to find the one that fits your playing style. Knowing a few alternatives does help.