Generally, companies filter out candidates who request to look at any measurable amount of source code as part of the process. Larger companies leveragethe 6-12 mo contractor to hire. You are still stuck there until you are not.

These topics are common knowledge, if you have interviewed in the last 5 to 10 years. I have been working for 25, so I find the blame trying to be redirected, by some, misguided.

Yes, you can't directly look at the source code (unless you pick companies that open source a lot). But I was more thinking of trying to develop some proxy metrics that you can measure; the most common being asking the right questions in the interview. But you can also try to look for other tells.