[throwawaykf05]

You are supposed to prove it during the lawsuit, but you can get one filed just on "belief" and vague accusations. This stays the lawyers bills running for the defendant while the plaintiff can just wait, which is a tactic often resulting in early settlements.

In the US there is something called rule 11 that prevents you from filing bad faith lawsuits, but it is a pretty low bar. IMO if this is strengthened by, say, requiring detailed claim charts up front, it can go a long way in deterring nuisance lawsuits.