Letstalkfish is spot on, it nitrifying bacteria is so small that its should be basically invisible.
THE ONLY WAY TO TRULY KNOW IS TO TEST YOUR TANK WATER! API makes affordable and mostly accurate test kits that measure ammonia/nitrite/nitrate/ph for around 25-30$! This IMO is extremely important to the long term success of any aquarium! Sure you can skimp and do without but why not make sure, plus you can measure you peak Bio load and schedule water changes accordingly so you don't over exceed recommended levels of toxins or overdue unnecessary water changes! Ammonia obviously should be zero, as well as nitrites, and IMO nitrates shouldn't exceed 30-40ppm! Pothos plants will help tremendously decrease nitrates and are a simple and cheap investment (check the link below)
Here are a few microscopic images of BB:
And just so you know the order of events when it comes to the nitrogen cycle:
And of course a generic link explaining the principals more scientifically then I can ATM!
http://www.theaquariumwiki.com/Nitrifying_bacteria
Hope all that helps and like I said testing your water is the ONLY 100% way to know you have both beneficial bacteria colonizing in your Bio media! I find API CHEM STARS are the most efficient but also designed mainly for canister filters!
Cheers
×Go S. Vettel #1 Infiniti Redbull! 4x WDC!!! Congrats on another flawless title×
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Cheap way to decrease nitrates and keep your fish healthy: http://monsterfishkeepers.com/forums/showthread.php?t=504763