The answer by Jon Skeet is correct.
In Joda-Time
Just for fun, below is the solution in source-code using the third-party libary Joda-Time 2.3 in Java 7.
Details
The DateTimeZone class has a method, isStandardOffset. The only trick is that the method takes a long, the milliseconds backing the DateTime instance, accessed by calling DateTime class‘ superclass‘ (BaseDateTime) method getMillis.
Example Source Code
// © 2013 Basil Bourque. This source code may be used freely forever by anyone taking full responsibility for doing so.
org.joda.time.DateTimeZone losAngelesTimeZone = org.joda.time.DateTimeZone.forID("America/Los_Angeles");
org.joda.time.DateTime theSecondAt6PM = new org.joda.time.DateTime( 2013, 11, 2, 18, 0, losAngelesTimeZone ) ;
org.joda.time.DateTime theThirdAt6PM = new org.joda.time.DateTime( 2013, 11, 3, 18, 0, losAngelesTimeZone ) ; // Day when DST ends.
System.out.println("This datetime 'theSecondAt6PM': " + theSecondAt6PM + " is in DST: " + losAngelesTimeZone.isStandardOffset(theSecondAt6PM.getMillis()));
System.out.println("This datetime 'theThirdAt6PM': " + theThirdAt6PM + " is in DST: " + losAngelesTimeZone.isStandardOffset(theThirdAt6PM.getMillis()));
When run, note the difference in offset from UTC (-7 versus -8)…
This datetime 'theSecondAt6PM': 2013-11-02T18:00:00.000-07:00 is in DST: false
This datetime 'theThirdAt6PM': 2013-11-03T18:00:00.000-08:00 is in DST: true
About Joda-Time…
// Joda-Time - The popular alternative to Sun/Oracle's notoriously bad date, time, and calendar classes bundled with Java 7 and earlier.
// http://www.joda.org/joda-time/
// Joda-Time will become outmoded by the JSR 310 Date and Time API introduced in Java 8.
// JSR 310 was inspired by Joda-Time but is not directly based on it.
// http://jcp.org/en/jsr/detail?id=310
// By default, Joda-Time produces strings in the standard ISO 8601 format.
// https://en.wikipedia.org/wiki/ISO_8601
// About Daylight Saving Time (DST): https://en.wikipedia.org/wiki/Daylight_saving_time
// Time Zone list: http://joda-time.sourceforge.net/timezones.html