i would think that if you can't remove the hygrmeter, you will need to get another one to check the calibration of yours. It is not that critical that it be calibrated as long as the cigars are burning right and you know what range the hydrometer reads for them to burn right. The problem comes when you first set up the box, you can get some pretty big swings in humidity until the moisture equilibrates. Radioshack has a pretty good digital with high/low memory that goes for about 20. You could get that and calibrate it and check the gauge you have. You'll probably be getting another box soon anyway.
Thanks for the feedback, I appreciate it! I'll see if I can find the ones you mentioned. For now, I picked a digital one up at Radio Shack for $20 and it looks like it's working well so far. It's reading from 71% to 76%; I look at it multiple times a day.