This would be same for all cities, everywhere. You need to know which areas of the city to avoid. Those areas where the poor and the desperate live: people who would see you as an opportunity to make some easy money - and who wouldn't care too much about how they took it from you.
@peterxdunn Your answer made me think: not just the poor and desperate see you as an opportunity to make money, but also quite a few of the restaurant etc owners in the most touristy areas. To avoid as well, so a guide should also advise you which are the honest localities! Good one!