The standard for validation is pretty low, but most debt collection agencies and law firms will not send a written response unless they are actually collecting the debt. After you know they have a debt that is yours, and the debt is within the statute of limitations in your state, then work on a settlement that you can afford. You should never just settle unless you are sure it is your debt they are legitimately collecting.
Threats and belittling may be violations of the FDCPA and maybe even the laws in your state are there to protect you from illegal harassment. Keep in mind, just because they may have filed suit does not mean they can win the case. There are many defenses and the requirement that they actual prove the debt with admissible evidence.