How to tell if JRE or JDK is installed

You can open up terminal and simply type

java -version // this will check your jre version
javac -version // this will check your java compiler version if you installed 

this should show you the version of java installed on the system (assuming that you have set the path of the java in system environment).

And if you haven't, add it via

export JAVA_HOME=/path/to/java/jdk1.x

and if you unsure if you have java at all on your system just use find in terminal

i.e. find / -name "java"

Normally a jdk installation has javac in the environment path variables ... so if you check for javac in the path, that's pretty much a good indicator that you have a jdk installed.