is a character already in the range

I think this from here will help you.

NSString* floatString = @"12.345";
NSRange dotRange = [floatString rangeOfString:@"."];
if (dotRange.loc = NSNotFound && dotrange.len = 0) {
    NSLog(@"floatString doesn't contain .");
} else {
    NSLog(@"floatString contains .");
}

There are many methods in the NSString class for searching strings.  You'll find the documentation here:

http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/NSString_Class/Reference/NSString.html

You can find the position of a substring with:
NSRange r = [string  rangeOfString:@"."];
if ( r.location != NSNotFound ) {
    bla bla bla
}

If you're simply wanting to convert that string to a double you can use the doubleValue method (and or intValue/longValue, boolValue floatValue for atoi type conversion).

double d = [string doubleValue];

If you know the stardard C library functions (strlen,strstr and so on), you can use them:
char* theBytes [string UTF8String];
if ( strchr(theBytes,'.') ) {
  bla bla;
}

I do NOT recommend using the C library functions, however they might help you transition from the familiar land of C to the much more charming (and slightly mysterious) world of Objective-C and Cocoa.