How to trace if the request fails before get to varnish or at varnish?

[Abraham Cruz Sustaita]

I have two varnish servers. If the request to the backend fails, varnish
throws an error page, but the problem is that now Varnish is radomly
failing. The connection expires and I see the error (at firefox) "The
server is taking too much time to respond". Is there any way I can debug if
the problem is with varnish or before?  I have one switch, one firewall and
one load balancer before varnish
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