The same way you learn if a job has bad management when considering taking a job there - reputation, sites like Glassdoor, asking friends in the industry, etc.

The same way you learn if a job has bad management when considering taking a job there - reputation, sites like Glassdoor, asking friends in the industry, etc. This problem doesn't seem unique to unions, nor does it seem that not having unions solves it. Management can still refuse to fire a bad employee in a non-unionzed shop. I've seen it countless times.