How do I know for sure IF I have Java 6 (from Apple) installed as well?
java -version | open -f -a TextEdit
java version "1.6.0_51" Java(TM) SE Runtime Environment (build 1.6.0_51-b11-456-11M4508) Java HotSpot(TM) 64-Bit Server VM (build 20.51-b01-456, mixed mode)
then the latest version of the Apple JRE is installed. Anything else, it's not the latest version, or not installed at all.
And would this Java 6 update affect my Java 7 install in anyway?
No. They're completely independent of each other.