Numerous studies say union workers make more on average even accounting for dues. I'll give you 1.[1]
Unions have to disclose financial statements. Leaders can't just raid the strike fund. And there won't be a strike fund unless most members vote for it.
If I'm reading that study correctly, those figures have an R² of around 0.23, which is quite low for explanatory power of wages as dependent on union membership (quite low as in the "none" or "very low" range)