From the command line I would use
jinfo < pid >
which will give you this information and more
You can fire up JConsole, connect to the JVM running Weblogic and then navigate to the "VM Summary" tab to inspect the classpath/vm args.
You can write a small application to connect via JMX and query the mbean java.lang.Runtime. It has an attribute "ClassPath".
import java.lang.management.*ManagementFactory.getRuntimeMXBean().getClassPath()