By looking at the contract.
Oracle does not technically limit installing and using features, anything can be installed, whatever you have in the database may not reflect what you are allowed to use. It is the responsibility of the DBA to not install or disable unlicensed features wherever it is possible.
Starting with 11g, there is a parameter called CONTROL_MANAGEMENT_PACK_ACCESS. Its default value enables both Diagnostic and Tuning Packs, but in a properly managed environment it should be set according to the license. When the value of the parameter includes
DIAGNOSTIC, then Diagnostic Pack (which includes AWR) is enabled, otherwise it is disabled.