Retrospective Software Validation is not considered Industry Best Practice. Get a copy of GAMP (Good Automated Manufacturing Practices) the latest version is 5 and can be found on the ISPE website.
I do not know of any standards for RSV because it is not a best practice.
You would have to talk to regulators or auditors.
Indeed, a retrospective validation (whether software or equipment) is not a best practice but due to a variety of factors, there are times when it needs to be done. I’ve never seen it documented or described anywhere but here’s my take. It’s a 2-step process:
Step 1 is to perform the validation. That’s a fairly well-defined process.
Step 2 is to assess the results with respect to prior use of the software. For example, if you found out that the software didn’t provide correct results for a certain range of inputs, you would need to assess what impact that had on your products or service prior to the validation. That could include anything up to product recall and/or production line stoppage if the severity is sufficient. Likely (and hopefully) you don’t find anything that severe. But you should make an assessment of the quality of product or service prior to validation regardless of findings. Document your actions, findings, and assessment.