I should amend my above statement to, "oxygen isn't a problem until primary fermentation is done". During the active stage of fermenting CO2 pushes oxygen out of the fermentor. Some people put a loose lid or even just a clean rag over the bucket until primary starts to slow down. Then rack to secondary with no head space. That's when O2 becomes the enemy.
You need a hydrometer. You can take a sample by removing the airlock bung and drawing with a wine thief or sanitized turkey baster. I rack to secondary when it gets below 1.010 specific gravity.