How do you use command line tool (CMD) to check SQL Server Report Servers?

You could use the command rskeymgmt

Here are the things you can find out about Report Servers from command lineusing HELP:
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C:\Users\TEMP.HODENTEK9.000>rskeymgmt -?
Microsoft (R) Reporting Services Key Manager
Version 13.0.1601.5 x86
Performs key management operations on a local report server.
  -e  extract           Extracts a key from a report server instance
  -a  apply             Applies a key to a report server instance
  -s  reencrypt         Generates a new key and reencrypts all encrypted
                        content
  -d  delete content    Deletes all encrypted content from a report server
                        database
  -l  list              Lists the report servers announced in the report server
                        database
  -r  installation ID   Remove the key for the specified installation ID
  -j  join              Join a remote instance of report server to the
                        scale-out deployment of the local instance
  -i  instance          Server instance to which operation is applied;
                        default is MSSQLSERVER
  -f  file              Full path and file name to read/write key.
  -p  password          Password used to encrypt or decrypt key.
  -m  machine name      Name of the remote machine to join to the
                        scale-out deployment
  -n  instance name     Name of the remote machine instance to join to the
                        scale-out deployment
  -u  user name         User name of an administrator on the machine to join to
                        the scale-out deployment.  If not supplied, the current
                        user is used.
  -v  password          Password of an administrator on the machine to join to
                        the scale-out deployment
  -t  trace             Include trace information in error message
To create a back-up copy of the report server encryption key:
RSKeyMgmt -e [-i ] -f -p
To restore a back-up copy of the report server encryption key:
RSKeyMgmt -a [-i ] -f -p
To reencrypt secure information using a new key:
RSKeyMgmt -s [-i ]
To reset the report server encryption key and delete all encrypted content:
RSKeyMgmt -d [-i ]
To list the announced report servers in the report server database:
RSKeyMgmt -l [-i ]
To remove a specific installation from a scale-out deployment:
RSKeyMgmt -r [-i ]
To join a remote machine to the same scale-out deployment as the local machine:
RSKeyMgmt -j [-i ] -m
          [-n ] [-u -v ]
C:\Users\TEMP.HODENTEK9.000>
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I have three report servers but two of them have a problem.

If you want to use rskeymgmt you should start the CMD program with elevated privileges as shown:

Once CMD screen is displayed you can find the report servers as shown. I am using the l and i flags in rskeymgmt. The SQL Server Report Server 2012 PCATT has no problem, while the 2016 and 2017 SQL Server instances OHANA and SSRS are showing the same exception.

C:\>rskeymgmt -l -i PCATT   ---SQL Server 2012
HODENTEK9\PCATT - 879ea471-47cf-4386-b7f1-6eb213a5fff6
The command completed successfully
C:\>rskeymgmt -l -i OHANA     ---SQL Server 2016
The profile for the user is a temporary profile. (Exception from HRESULT: 0x80090024)
C:\>rskeymgmt -l -i SSRS      --default instance of SQL Server 2017
The profile for the user is a temporary profile. (Exception from HRESULT: 0x80090024)

The previous result was obtained using a Preview Build of OS that was not working well.

Recently 9/14/2018 there was an update to the OS and things have improved. Here are some new results for the same. 

Two of them are not displaying results, in one case the Database Engine is not running and in the other the Report Server is not running.