By slack do you mean high margin or high revenue per employee or something similar? Or a large war chest/lots of runway?

Definitely, large war chest and lots of runway always helps. Also high margin may be good or bad but high revenue per employee(i.e. consulting firms in government, defense, finance, energy etc. sectors) are the most human friendly places to work at.

Do you have any actual script, as in, exact questions to ask during interview, so that executives and/or techies would answer clearly whether their company has financial slack?

Revenue or profits per employee is a good start.

Just curious… How do you find out there is slack before actually getting the job?

One hint might be found in yearly financial reports. It depends a bit on the country if you can access those reports from non-publicly traded companies. Are they debt-heavy, or do they have enough free cash flow? What if all customers disappear the next day, can the business continue for an extended period of time?

Is that indicator of slack though?

relevant:

Slack, by Tom DeMarco https://www.amazon.com/Slack-Getting-Burnout-Busywork-Effici...