How do you know if you need an umbrella liability policy?

If you have lots of assets to protect, a future to ensure (kids, retirement investments), multiple homes,vehicles, animals, anything that can harm anything/anyone inadvertently or through negligence or in any way shape or form, insure everything, that's what an "umbrella" does: it covers everything. You never know how good your insurance policy is until you need to use it; if you have lots of things to lose in a lawsuit or in a disaster, insure accordingly.