You can always use
UIFont *systemFont = [UIFont systemFontOfSize:12];
NSLog(@"what is it? %@ %@", systemFont.familyName, systemFont.fontName);
The answer is:
Up to iOS 6
Helvetica Helvetica
iOS 7
.Helvetica Neue Interface .HelveticaNeueInterface-M3
but you can just use Helvetica Neue