By using the iOS simulator that comes with the (free) iOS development kit you can achieve a lot; but profiling is generally tricky. Run it with the debug console; but do the major testing in Safari. (Not Mobile Safari, but the desktop version.) This should help you root out most actual errors.
As for performance, there's not a lot more you can do than to simply test it on a number of representative devices and see if it's fast enough. (2nd gen. iPod Touch, iPhone 3G, iPhone 3GS etc, with both iOS 3 and iOS 4 (latest versions) should be a good starting point.)