As you've not posted in a specific standard, in general, you'd verify the plan can be effective. You've mentioned CAPA and also "Corrective action plans". The two are actually different. CAPA stands for corrective action, preventive action and is so two parts, the latter is only half of the message and it's important to split out the two. In general, when I see CAPA they still only have a correction not a prevention. A correction solves that specific issue, the prevention is intended to make sure it never happens again. The only way to have a decent preventive action is to give at least some thought to the root cause(s) of why the thing went wrong in the first place. So my first step if I was an auditor is to look at the proposals, has root cause been considered? Is the prevention likely to address the root cause?
If you're then asking how you verify the actions are complete, there's no substitute to looking in person and especially talking to people about it. If you're an external auditor this is virtually impossible unless the audit was very poor and sadly you have to rely on a bit of trust in the supplier. Normally you'd ask for evidence, e.g. training records, photos and that's all you can do to verify the action has really been closed out.
