How do I test cats to see if there bad

It is most likely that 'YOU' can't !! ... Using professional scanner > live sensor data to monitor how often upstream and downstream O2 sensors are switching and what each sensors value is...or by using an exhaust gas analyzer > (tail pipe tester) .. Long term, Improper ignition timing may contribute to catalyst failure but is not what is throwing 420 & 430 codes! .. Keep in mind the vehicle IS 17 years old! .. Have 'qualified' auto repair shop > mechanic test it... It is well beyond DIY diagnosis!