I'm not proud of this, but it's the cleanest answer available to me right now.

stret.m

// It would be a mistake to use this code and expect it to work at any point in the future.
NSNumber *encoding_requiresStret(const char *encoding) {
    NSMethodSignature *signature = [NSMethodSignature signatureWithObjCTypes:encoding];

    // This information is obviously available internally, but it's only publicly exposed here.
    NSString *signatureDescription = [signature debugDescription];
    
    BOOL isStret = [signatureDescription rangeOfString:@"is special struct return? YES"].location != NSNotFound;
    BOOL isNotStret = [signatureDescription rangeOfString:@"is special struct return? NO"].location != NSNotFound;
    
    if (isStret) {
        // objc_msgSend_stret will be used to invoke methods with this signature
        return @YES;
    } else if (isNotStret) {
        // objc_msgSend will be used to invoke methods with this signature
        return @NO;
    } else {
        // This is so fragile, I'm not even sure how I sleep at night. This is why we can't ship nice things.
        return nil;
    }
}

YES/NO/MAYBE