Command to check if open proxy works

Hello,

please what is the least server resources intensive linux command to veriffy an open proxy works.

I mean one should probably send and receive any data via this proxy? and if data found it returns true

Assuming proxy is 1.2.3.4:1080

I used this: curl -x $proxy $url --connect-timeout 8 --max-time 20

but it appears my proxy timeout, but still connect, not sure if after timeout no proxy is used, so i want to load via proxy or return false

You can use telnet to check proxy connection, like following.
telnet: > telnet proxy.example.com 8080
telnet: Trying 192.0.2.2...
telnet: Connected to 192.0.2.2.
telnet: Escape character is '^]'.
client: CONNECT mx1.example.com:25 HTTP/1.0

server: HTTP/1.0 200 Connection established

server: 220 mx1.example.com ESMTP server ready Thu, 5 Feb 2004 00:38:22 +0200