Procedure for experimentally determining decoupling capability?
Ok, ignoring your esd issue. Use the scope for sure and a high frequency signal generator capable of delivering up to 1GHz sourced from 50 ohms.

It probably would be better with a spectrum analyser the could handle up to 1GHz though.

Inject small signals onto the logic rails at various points and any weaknesses will be seen by the scope/analyser. 100mV p-p will do the job.

Also move the scope/analyser to different points while injecting. See if there are some frequencies that are more prone to generate bigger disturbances than others.

You might even be able to setup a pulse generator to do a similar job to the Sig gen. Stick with 100mV though to avoid damage.