How to determine if google public stun server is alive or usable? [closed]

I have seen a lot of of WebRTC client libraries using public STUN server like below:

"stun.l.google.com:19302",
"stun1.l.google.com:19302",
"stun2.l.google.com:19302",                                                                                                                              
"stun3.l.google.com:19302",
"stun4.l.google.com:19302",

I am wondering if the servers above are dead? Because I have tried:

telnet stun.l.google.com 19302
telnet stun.l.google.com 3478

I get the following response:

telnet: connect to address 74.125.204.127: Operation timed out
telnet: Unable to connect to remote host

Same thing happens to other google STUN server in list.

I couldn't get any response, does this mean the STUN servers from Google is not usable anymore?

My question is: Is telnet like the above the correct way to determine if a service is alive or not?

I receive a reply when sending a STUN packet to port 19302 on stun.l.google.com.

Your telnet command gets no reply because it is not speaking STUN. It will attempt to open a TCP connection, which fails because STUN is running over UDP not TCP.