./bin/server version
It will show you what version of liberty is running, as well as which version of the JDK/JRE the server is configured to use.
The output (I'm on a mac), would look something like this:
WebSphere Application Server 8.5.x.x (1.0.x.xxxxxxxxxx) on Java HotSpot(TM) 64-Bit Server VM, version 1.6.0_65-b14-462-11M4609 (en_US)