Without the Hint, how can I know it's programmed to accept Ronald?

In general: no.

In the very rare case you have access to the source code or more, then yes, probably.

You will always need some indication from somewhere of what is needed.

What do you mean by rare case when you have source code, I thought that it is always available.
Also it’s possible that this type of feature could exist, say in Amazon website, but only those with access to its database would know about it and it would be used as shortcut right?

You only have the source code of the make-up of the pages you view in your browser. These normally do not contain any crucial/secret logic. The source code I’m talking about is the code on the server itself, which generates the pages you view and would contain checks such as the one you asked about. This code is normally not available from the outside.

For example, you will not find the check for ‘Ronald’ in the source code you have for this challenge. Just the code to select a user and submit it to the server.

And yes, anything is possible, but I doubt Amazon would have something this silly.

So what would be the logic in finding out the sites logic? Guessing? To figure out weaknesses?

Thanks!

Yes, observing its behaviour and/or finding weaknesses.