code: func (sm *Manager) SessionIsActive(ctx context.Context) (bool, error) {
  	if sm.userSession == nil {
  		return false, nil
  	}
  
  	req := types.SessionIsActive{
  		This:      sm.Reference(),
  		SessionID: sm.userSession.Key,
  		UserName:  sm.userSession.UserName,
  	}
  
  	active, err := methods.SessionIsActive(ctx, sm.client, &req)
  	if err != nil {
  		return false, err
  	}
  
  	return active.Returnval, err
  }
docs: // SessionIsActive checks whether the session that was created at login is
  // still valid. This function only works against vCenter.