Well, the best way to tell is the use of the words ahou and baka. Both can mean idiot, but one can be more insulting than the other, depending on where you are. In Tokyo, if you call someone a baka, then they won't take it as much of an insult. But if you use ahou, they'll be like, "What did you say, dirtbag?!" However, with Kansai-ben, the usage of baka and ahou are reversed.