One of the things you will find about auditing is that you will develop your own method of approaching an audit. As is noted above, the way you audit will in part be a function of the purpose and scope of the audit. In the case of a customer auditing you (or you auditing a customer) there will probably be a focus on that product and related paperwork alone whilst in an ISO registration audit, the auditor will be most interested in the systems in general. They may track a single product through all the systems, however the focus will be on the systems, not so much any specific product.
On the other hand a product audit may consist only of taking a sample and inspecting them.
Internal audits are another matter entirely.
There are may audit strategies . Which you use will depend upon your personal methodology as well as the scope and intent of the audit. Take for example Up Stream and Down Stream audits: Both of these audits are simply where one starts at one end and finishes at another.
Take a packaged product ready to ship and start working backwards. You can eventually reach the purchase order for that product.
Take a request for quote or other early document (such as a PO) and follow the process. For example, one might want to start by asking to see evidence of review of the RFQ or the purchase order. Next, let s see the job registered in the planning system. Etc.
ISO 19011 - Quality and Environmental Management Systems Auditing Forum Discussions