NSString test for only numeric characters

[josh_feinstein]

I have an NSString object that I would like to test to make sure that it
contains only numeric characters and no alpha characters. The test will
happen almost everytime the user makes a request to the app, so I am
trying to decide which is the tightest, quickest, most effiecient method
to use. Should I convert the string into an NSNumber or NSArray or
NSScanner class and what is the "best" method within one of these three
classes to use?

Thanks,
Joshua