Once my tank finished cycling, before I added fishies, I checked the tanks ability to rid itself of ammonia like this....
Once your ammonia level has gone to 0, nitrites are 0, and nitrates are up to 20-40 or so.....
Do a water change to get your nitrates to around 5.
Then introduce ammonia back in the tank as you did the first time.
Check the water in about 24-36 hours.
The bacteria, if they are doing their job, should have converted the ammonia rather quickly (Ammonia 0, Nitrites 0, Nitrates >10). This will serve as a "ready test" for your tank. The last thing you want is to "think" your tank is cycled, and it's really not. This will be the true test.
IF all is successful, and the ammonia was quickly converted, do another water change and go look for some fishies!
I will give you a head start, do some research on "Diatoms"..they will be next on your list of things you will want to know about. :o)
Enjoy, coolest hobby on the planet. Good luck!