How do you check transmission oil on 2002 harley Davidson 1200 sportster?

You normally won't ever "check" the trans oil. The transmission oil doesn't have a path near the combustion chamber so it will not be subjected to the same breakdown as the engine oil. That being said, when you replace the engine oil, it is only a little extra effort to replace the transmission oil. This will give you the opportunity to check the magnetic tip on the plug for any possible gear chunks signaling a future transmission breakdown.

If you really want to check it, you have to remove the derby cover (the larger dome cover on the primary), making sure not to strip out the buttery-soft Allen screws, and not lose (or damage) the small rubber gasket under it. The fluid should come up to just above the bottom of the clutch basket.