How to know JAVA version on the system?

For Windows;

Press Windows button + R and press cmd then click "OK".
Write down "java -version" and press enter

For Linux;
Open Terminal and write down "java -version" and press enter

e.g.
java version "1.7.0_55"
Java(TM) SE Runtime Environment (build 1.7.0_55-b14)
Java HotSpot(TM) Client VM (build 24.55-b03, mixed mode, sharing)