I am not aware of a scheme to validate the keys.
The safest approach would be to import them in a new Freenas installation in the event the keys are not correct.
To do that, you can shut your system down, remove and place your boot key aside for safekeeping.
On a new boot medium, install Freenas. Make sure your encrypted volume are not physically connected as to prevent any accidental overwrite.
Boot with the new Freenas environment. Everything will be as new. Shutdown system and connect encrypted drives.
Boot system and import encrypted volume following normal step with geli file and verify import is complete and successful. It may take quite some time if yiu have lots of data on the volume.
Then detach the volume and proceed to the import with geli-recovery key. Both should work.
You can follow those steps on a different system if you have one and the outcome should be the same.
When everything looks fine, shutdown your system and replace the boot drive as it was before the test. Everything should now be as before.