how can I test coils?

I have check engine light and had the code checked got code P0430(cat efficiency low bank 2)now I am pretty sure that at least one of the coils and a leaking gasket are the problem.
But if some one can instruct me on how to test the coils to see which are bad that would be appreciated thanks.

Often if you have a bad coil, you'll also see a P030x code where x refers to the cylinder number with the misfire. Since you don't mention that you have a misfire code, some scan tools have the capability to monitor the component parameters. The component parameters will show misfire counts on the individual cylinders, even if the count is not high enough to trigger a misfire code.

Do you have the V6 or the V8? If the V6, you could fairly easily test 3 of the coils using an oscilloscope, but the bank 1 cylinders will be be nearly impossible to test with the engine still able to run. On the V8, all 8 are pretty easy to get access to.

-Rod

Its a v6 and i am going to check plugs tomorrow was also told it could be o2 sensor in rear

According to the factory service manual, possible cases of the P0430 code are:

Use of leaded fuel
Damaged HO2S
Malfunctioning ECT
High fuel pressure
Damaged exhaust manifold
Damaged catalytic converter
Oil contamination
Cylinder misfiring
Downstream HO2S wires improperly connected
Damaged exhaust system pipe
Damaged muffler/tailpipe assembly
Retarded spark timing-Rod