While checking on an ASL20 license header itself is supported by Apache Rat (duh), properly checking an applied ASL20 license (for usage outside the ASF) isn't supported yet.
I've created an AppliedApacheSoftwareLicense20 check which does check this, using the http://apache.org/licenses/LICENSE-2.0.html#apply template as template (duh).
This header matcher itself extends a more generic CopyrightHeader matcher, which only takes care of the copyright header line validation, using a regex pattern + configurable copyright owner.
As an example the following configuration:
will properly match a license header like:
Copyright 2012 FooBar (www.foobar.com)
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
I'll attach a patch with the two header matchers + unit tests shortly.