If your boss goes around telling you that you should keep the amount you earn private lest your colleagues get envious of you, you probably need a Union.
If you see people being hired on Agency contracts working the same hours and jobs as a permanent worker, but on a lower wage with less benefits, always getting fired before the AWR kicks in at 12 weeks. You should probably form a union before that becomes you.
If the company spreads "how to prevent unions forming" and "how to stop unions before they become a problem" among the management and only the management. You need to make a union.
If there a legal objections in the Union which force idiotic situations like only disallowing managers from using dollys, you should talk to the Union about getting rid of that.
If the Union actively antagonizes and practises nepotism. That Union needs to go.
If the Union hands you back your dues at the end of the month and tells you that it is under the table and you shouldn't write it on your taxes, its being used to launder money and may escalate to activity.>TL;DR
If you don't have a Union, unethical people may push you towards unionizing.
If you have a Union, it will become corrupt unless checked.