Since then, I have a rule, that only failed me once: before working a month for a person, work a week or less, get paid.
There are smart scammers who are really devious but those I have met had all kind of pathological problems they were in denial with. I think that most scammers live in a fantasy world.
By forcing them to prove they have money, are ready to pay you and know how to make a bank transfer, you are pruning a lot of them.
Also, and especially in my case where I have several international clients, the "I dont understand the bank transfer got refused" excuse is very common (and sadly, very believable). Having worked through all the troubles of making a payment from their accounts to yours already sets expectations about normal delays.