You can compare crime rates in one city before handgun carry to after handgun carry. And then you can see whether that rate of decline is more or less than the national average would have predicted. I dont have the time to do the research but I believe the answer is (having seen it quoted elsewhere) that crime declines more in areas with handgun carry than in areas without it.
Yes you can do that - but you also have to factor in other crime prevention changes that the city put in place because any one of them or all together may be responsible. Detroit is a particularly good example of this because they've taken very strong measures to reduce crime and they've had strong results.