I guess the rule of thumb here is just not to trust anything that promises you money for not doing anything. Promising to double your money if you jist send them money is sketchy enough. Never trust too good to be true gains. Ever.
Yes, in things like this sometimes we also have to use logical thinking so as not to get caught up in sweet promises from people we don't know, because in reality it is very rare for people to make promises to give big money to other people who she doesn't know him at all, so obviously on this point I totally agree that we all don't believe anything that promises you money for doing nothing.