You have to use the getState()
method of server runtime mbean.
You can obtain the server runtime mbean by navigating into wlst runtime tree or by using a lookup method.
Sample:
domainRuntime()
slrBean = cmo.lookupServerLifeCycleRuntime('Server1')
status = slrBean.getState()
print 'Status of Managed Server is '+status
See also Getting Runtime Information in WLST official documentation.