As Mr. Wizard mentioned, you cannot tell if it can be trusted or not. This is why its good to keep an equipment log noting when important stuff like rope that you depend on is used and anything that happens to it.
However, if you are given some paracord sealed from a manufacturer and you are curious if it is the Mil-spec stuff or not then there are a few things you can do. Originally Mil-Spec Cord has a braided sheath that with no core is rated to 300 lbs. Open the sheath and count the yarns inside there should be 7 and each should be 3 twisted together. Each yarn is rated to just over 35 lbs. On most commercial stuff the inner strands are just 2 yarns twisted together.
If you are still unsure then get a Force Gauge and test a single section of the rope. Remember though when testing a single section of rope it tells you NOTHING about the rest of the rope, just if that section was up to the standard. It also means that section of the rope should be disposed of because it has been taken up to it's load and probably cannot be safely done again. If you are going to go the testing route and you need to buy some of this crazy expensive equipment might I recommend a Universal Force Test System as it will test more than just pull tension.