How can I deduce from NSMethodSignature that a struct argument is passed by pointer to the stack?

Specifically on ARM.

For example if I have:

@protocol TestProtocol <NSObject>
- (void)time:(CMTime)time;
- (void)rect:(CGRect)point;
@end

And then I do:

  struct objc_method_description methodDescription1 =
    protocol_getMethodDescription(@protocol(TestProtocol), @selector(time:), YES, YES);
  struct objc_method_description methodDescription2 =
    protocol_getMethodDescription(@protocol(TestProtocol), @selector(rect:), YES, YES);

  NSMethodSignature *sig1 = [NSMethodSignature signatureWithObjCTypes:methodDescription1.types];
  NSMethodSignature *sig2 = [NSMethodSignature signatureWithObjCTypes:methodDescription2.types];

  const char *arg1 = [sig1 getArgumentTypeAtIndex:2];
  const char *arg2 = [sig2 getArgumentTypeAtIndex:2];

  NSLog(@"%s %s", methodDescription1.types, arg1);
  NSLog(@"%s %s", methodDescription2.types, arg2);

The output is:

v40@0:8{?=qiIq}16 {?=qiIq}
v48@0:8{CGRect={CGPoint=dd}{CGSize=dd}}16 {CGRect={CGPoint=dd}{CGSize=dd}}

Both look similar, no indication that CMTime will be actually passed as a pointer.

But when I print the debug description:

  NSLog(@"%@", [sig1 debugDescription]);
  NSLog(@"%@", [sig2 debugDescription]);

The first prints:

  ...
  argument 2: -------- -------- -------- --------
        type encoding (^) '^{?=qiIq}'
        flags {isPointer}
  ...

While the second prints:

  ...
  argument 2: -------- -------- -------- --------
        type encoding ({) '{CGRect={CGPoint=dd}{CGSize=dd}}'
        flags {isStruct}
  ...

So this information is indeed stored in the method signature, but how do I retrieve it without parsing the debug description? Are there rules I can use to deduce this myself? I tried different structs but it is hard to spot a pattern.