You have to check the gravity to see if it is finished fermenting. Check it today, wait a few days and check it again. If the gravity does go down between the two measurements, wait a few more days and check it again. If it hasn't budged and it is reasonably close to your target final gravity, it is done and you can rack to secondary.
I would check the final gravity the same way you checked the original gravity. Personally, I use a sanitized turkey baster and pull off about a 4 ounces of beer, pour it into a testing tube and insert the hydrometer. Don't pour the beer back in, just drink it.
I wouldn't worry about oxygen, there should be a blanket of CO2 in there. Just don't move it around or splash.