Before you get offers, you are looking to disqualify companies, ie: What red flags can I notice that make me reasonably confident that I would never want to work at this company. After you get offers, you are looking to qualify companie, ie: What green flags can I notice that make me reasonably sure that I definitely want to work at this company.

Depending on your confidence level going in about how many offers you expect to get and how many offers you actually end up getting, your qualification and disqualification steps should be more or less rigorous. If you're confident of getting many good offers, then ask the tough questions in the phone screen to avoid wasting your time.