How to know if a UITextField in iOS has blank spaces

You can “trim” the text, that is remove all the whitespace at the start and end. If all that’s left is an empty string, then only whitespace (or nothing) was entered.

NSString *rawString = [textField text];
NSCharacterSet *whitespace = [NSCharacterSet whitespaceAndNewlineCharacterSet];
NSString *trimmed = [rawString stringByTrimmingCharactersInSet:whitespace];
if ([trimmed length] == 0) {
    // Text was empty or only whitespace.
}

If you want to check whether there is any whitespace (anywhere in the text), you can do it like this:

NSRange range = [rawString rangeOfCharacterFromSet:whitespace];
if (range.location != NSNotFound) {
    // There is whitespace.
}

If you want to prevent the user from entering whitespace at all, see @Hanon’s solution.