While spending time with Varnish I started a note pad with my notes and lessons of how to work with Varnish. I should say I am still new to Varnish and my notes are by no means a best solution for the problems they address, but I found myself refering back multiple times.
To get some information on what is going on in Varnish, I have used the following methods for help in these matters:
View the headers
It’s nice to view the headers of the page or resource to determain some fields that may have been set from Varnish to help with troublesooting. Or to show which cookies you might need to remove.
ULRs hitting the backend and not getting cached
If you need to view the requests that are not getting cached, this command helps. There are two ways to view this list, one that is constantly updating, and the other that prints out the top entries and then exits.
Test varnish configuration script for errors
Sometimes making changes to the VCL configuration can results in error. Find out where the error is with this command: /usr/sbin/varnishd -C -f default.vcl