Lawyers/Law Students: what's involved with proving a defendant acted in "bad faith"?

My parents are included in a group that are suing a guy and his family members for using his company as a vehicle to siphon out their investments for the benefit of the insiders. I’ve been reading up on what’s involved with piercing the corporate veil and establishing alter ego liability online, and one of the requirements seems to be establishing that the defendant acted in bad faith.

My question is what’s involved with meeting this requirement? does the plaintiff need to present evidence that actually proves a malicious thought process (like e-mails, or a journal entry that outlines the person’s thinking?) or would simply having the accounting records of obviously shady/fraudulent transactions be the kind of thing that’s sufficient (e.g. borrowing money from the company at very low interest rates, and then reinvesting that borrowed money in the company and giving yourself exorbitant rate of return like 50%)? From the few undergraduate courses I took on law, I recall that establishing intent can be particularly tricky. Is proving bad faith the same kind of challenge that would be involved with proving intent in criminal law?

I’m not looking for actual legal advice, just to help understand what’s involved, in the general sense, with establishing that someone acted in “bad faith”.