How to effectively audit training and skills development?
You can have two kinds of audits: conformance or performance audits.
With conformance audits you want to check if the standard and or internal procedures are followed.
With performance audits you want to check if the training and skills development investments are effective. For example, your organization may provide training to reduce product nonconformity or improve productivity. Your performance audit can check if actually training had any impact on product nonconformity levels or productivity levels.
What would be the audit criteria?
For conformance audits you have as audit criteria, the standard and your organization’s internal procedures. For performance audits you have as audit criteria the training objectives.
Should there be a training plan in addition to training records?
Although not mandatory according to ISO 9001:2015 it is a good practice that I recommend
The following material can provide more information