In Weblogic console
Environment --> Servers --> choose applications --> Monitoring --> Health/Performance/Threads/WorkLoad
Probably these in built weblogic console features help you in monitoring the health/status of the applications deployed.
Best option would be to have the OEM configured, which can provide details about all the clustered weblogic server environment.
Do we have any chance to verify from Linux command-line ? Any command ?
I don't think we have any OS level tools to check the load in particular, but, we can definitely use normal OS level commands like, top, nmon and check for the managed server load as they are basically java processes.
Hope this helps.