Well, you have to keep in mind that most pre-modern societies had very very small financial sectors, and involved far fewer people. So it was harder to operate anonymously in those sectors. Also, pre-modern financiers tended to be much more conservative in terms of who they loaned to, the terms they would lend at, etc. 

A good deal of fraud and embezzlement actually took place in the core agricultural sector, however. Fudging the accounts, making sick animals look healthy and vice versa, adulterating quality of products so that you can skim off the top, literally putting your thumb on the scales when it came to weighing goods, etc. 

So how did they deal with it? Well, because of the centrality of agriculture, there was a pretty sophisticated system of food inspection, and fraudulent bakers and other vendors were publicly humiliated by dragging them around the streets (either on a sled or hurdle, which added a measure of public humiliation) with their adulterated goods hanging around their neck, so that the consuming public knew who not to frequent. The pillory was frequently used to punish commercial fraud, as both a method of informing the public and as an outsourcing of physical punishment to the crowd.  

And those were relatively light punishments for financial crimes – if you were caught committing forgery, the penalty was death by either boiling oil or by having molten metal poured down your throat. In England, after 1278, stealing over four pence worth of goods was punished with the hangman’s noose. And so on and so forth.