Two Hydrometers - Two Readings

I just took a sample out of my primary and took down my readings. Just for the heck of it, I decided to use another hydrometer I haven't used yet and the readings were six points different! For consistency purposes I am going to use the hydrometer I originally used for the original gravity, but is there a way to tell which hydrometer is the accurate one? I'm going to try putting them in water at 60 degrees and see if the measurements are off.
The "old" hydrometer has been used for at least a year or so, but has never been boiled or dropped (or at least not that I know of).

Are they both calibrated to be read at 60? Some are different. Check that first.

Past that.. Some will be off.. Just compare them in some clean, distilled water and see what they read. From there you can adjust for any readings on each one.