All supported DevTest releases.
The license file resides in the DEVTEST_HOME folder.
First you must make sure the <validuntil> in the devtestlic.xml file has not expired. ex: <validuntil>12/31/20 00:00:00 -0500</validuntil>
Second, start the Enterprise Dashboard, this component is what verifies the license is valid. If it starts with no issues, then the license is valid.