How can I find out if code is running inside a JUnit test or not?

First of all, this a probably not a good idea. You should be unit testing the actual production code, not slightly different code.

If you really want to do this, you could look at the stacktrace, but since you are changing your program for this anyway, you might just as well introduce a new static boolean field isUnitTesting in your code, and have JUnit set this to true. Keep it simple.

It might be a good idea if you want to programmatically decide which "profile" to run. Think of Spring Profiles for configuration. Inside an integration tests you might want to test against a different database.

Here is the tested code that works

public static boolean isJUnitTest() {  
  for (StackTraceElement element : Thread.currentThread().getStackTrace()) {
    if (element.getClassName().startsWith("org.junit.")) {
      return true;
    }           
  }
  return false;
}