Why do you think you need to?

No.  I can be running any of a variety of
JRE's on my system, depending on the options
selected in browsers that happen to be showing
web pages with applets, apps. running under the
'default' JRE*, or apps. launched using web
start (which can also request both a minimum
version, or a specific version).

* Which I believe is what you might
be thinking, in asking that question.

The code ..

 System.getProperty("java.version");

..will return a String that represents the
Java version that the particular application
(or applet) is running in.

Here is a little app., that reports that
(and some other system properties).
<http://www.physci.org/jws/#jtest>

HTH

Andrew T.