You should test the pen when you receive (I did it once, for one pen but have rolled the dice since then with my growing pen collection.
To test, fill a cartridge with water and prime it to make sure the plunger is in the proper position for a dose. I injected/squirted into a water bottle cap and sucked up the expelled water with an insulin syringe to measure.
The last time this topic was discussed on this forum a user recommended inserting an insulin syringe into the pen (instead of using a pen needle) and inject directly into the syringe. Do this for a few different size doses (20u, 50u, 60u etc) and measure the result.