A prototype that has not been tested has not actually answered its question
It is easy to treat a functional prototype as finished once it prints successfully and looks right, but printing successfully only confirms the geometry sliced, not that the part actually does its job. A genuinely useful test plan starts from your specific concern, what you are actually worried might go wrong, and builds a short, concrete sequence to check it, rather than a vague intention to "test it later" that often never happens before the part ships or gets used.
How the plan is built
Four common failure modes, breaking under load, wearing out over cycles, not fitting, and permanently deforming, each get their own minimal test sequence built around isolating the specific feature of concern on a small coupon before committing the full part. Your own stated pass criteria and target cycle count or duration are carried through the plan explicitly, since this tool intentionally does not supply a generic threshold for what counts as "passing."
Worked example
Primary concern "breaks under load," pass criteria "holds 5 kg with no visible crack," no specific cycle target: the plan recommends isolating the feature on a test coupon, loading it well below 5 kg first to confirm no damage, then stepping up toward and past 5 kg while recording where visible stress or cracking begins, and repeating on a second coupon to check consistency.
A common mistake
A frequent error is testing the final part directly instead of a smaller, faster coupon that isolates the specific feature of concern. Testing the full part is slower to iterate and muddier to interpret, since a failure could come from several different features at once. A focused coupon test is almost always the faster path to a confident answer.
Limitations
This turns your stated failure mode into a minimal test sequence and records your own pass criteria; it does not supply pass/fail thresholds itself, since a generic threshold would be a false certainty this site avoids by design. Define your actual requirement before testing, not after.