The minimum insurance required to rent a car in the United States (or Canada) is basic public liability and property damage. (This covers only other persons and property, not the vehicle itself in the case of loss or damage... that is a separate coverage.) Read more about Car Rental Liability Insurance .
Car rental companies in California DO NOT include any state-required liability insurance into the base price of their rental cars. Most other states and provinces, DO include this coverage in the base rate of the rental.
You would need to check with your own personal automobile insurance provider to see if you either have the minimum insurance required to rent a car or, if not, if you would be able to purchase a non-owner liability policy that would cover you while in a rental car in another country.
If neither of these options is applicable, you will have to purchase the liability waiver or supplement available from the rental car company.
You also say that ..." if I have a smash I need to know I wont have to pay out anything (except a reasonable excess)." If you determine that you are covered by your personal auto insurance coverage, then make sure to check out what your deductible, or excess, is. The only way to avoid paying anything as far as an excess or deductible, is either to purchase separate collision or loss damage waiver coverage from a company that sells travel insurance and travel insurance options.
... or to purchase the optional loss and/or collision damage waivers offered by the car rental company.
You should also check your credit card to see if it offers rental car collision coverage as part of it's member benefits. If it does, make sure to check all the terms, conditions and restrictions of coverage (ie. will it cover you in a country different than your country of residence, is there a deductible, is it secondary to your personal insurance, etc).
Pages from this site, relating to this topic:
Car Rental Insurance Overview
Personal Auto Insurance Coverage for Car Rental
CDW and LDW Coverages
Car Rental Credit Card Coverages
Personal Protection Offers