I was not sure whether this should be an answer or a comment, but a good breath of fresh air cleared my mind:
You cannot guess rating based on short-term, small sample-size observation.
That is simply not what a rating is.
A rating is the manufacture saying
I promise that within X years, not more than Y‰ of our devices will malfunction by a deviation of D from this set S of specified properties, if operated, stored and transported under these environmental conditions E.
- X will be in the order of years. If you test only for hours or days, it will be hard to infer anything, unless your caps die horribly. Then you know "way to much", but nothing else, really
- Y is something that you'll need to define yourself. What failure rate is acceptable? Nothing in your question indicates you've given that any thought!
- apropos failure: what is a failure? Fire? A reduction of capacity by 10% A two-fold increase in leakage current? An increase in ohmic resistance? Susceptibility of any parameter to air humidity? D and S would answer that, and your application would define those, but again, no trace of consideration
- only one small part of E is what you seem to care about, but that's simply a misunderstanding of what it means to have a rating for anything.
Generally, unless very expensive components got accidentally packaged wrongly by accident (never saw that happen), there is no good reason to scrape off markings on a package. I've heard of counterfeit components, which someone tried to import, so they scraped off the markings and imported them as "no-brand". Those devices are either stolen, or actually of inferior quality. You would never use them - capacitors are a commodity device, so just throw yours away and get well-rated ones.