I don’t know how true this is across the board, but I’m an office worker for a power company and a lot of the people in my department are union because we work with the stuff around what the electricians and gas guys do. So maybe it’s worth looking into jobs that support the trades to see if the workers around such are unionized as well?

For what it’s worth, and more specificity, I do GIS mapping for electric and gas utilities in the area my company covers.