Arun,
When you install Javawebstart on a client machine, it installs a JRE along with it. JNLP would be meaningless without Java Web Start installed in it (.jnlp files would not be associated with anything in your system). So, assuming that Java Web Start is installed in the client machine, a JRE should also be present in it..reasonably speaking.
If you want to detect if the client machine has Java or not..one way would be to write an application that would scan the system registry (assuming your client is a NT machine) for the
word "VM" or "Java". (Alternatively, u can just do a 'System.getProperty("java.version") ' in your code and see what it returns). If it does not exist, start the Java (JRE) installation.