You've been asked to test a non-transparent proxy lo make sure it is working. After confirming the browser is correctly pointed at the proxy, you try to browse a web site. The browser indicates it is

A hung proxy would likely show no activity or an error state rather than a valid request captured and waiting in an intercept queue.

Trapping responses would still allow the request to be forwarded to the server - the response would arrive at the proxy and only be held before delivery back to the browser, which is a different failure mode.

When a proxy such as Burp Suite is configured to intercept (trap) requests, the outbound HTTP request from the browser is captured and held at the proxy, never forwarded to the destination web server. Because the server never receives the request, no response is generated and the browser waits indefinitely. This explains why a valid request is visible in the proxy yet the page never loads.

If the site were down, the proxy would receive a connection timeout or error from the remote server, not a cleanly held local intercept request.