How do I tell if I would benefit from a Squid?

Is there a good way to tell if my network users would benefit from a squid cache?
I assume setting one up and looking at it's logs would tell me, but at that point I may as well finish.
The network already has a Linux based NATing Firewall.

Depends on users and the content they're hitting. If they're hitting a lot of static content, the proxy will help. If it's dynamic information, not so much.

The only way to really tell is if you tried putting one in and saw what kind of traffic you're seeing. There's really no test that I know of to determine if you're in need of or could benefit from a proxy server.

You could try looking at your bandwidth use and if it's coming from a lot of HTTP traffic you should try the proxy. Also if your users are complaining about slow browsing then you can try the proxy.

If slow webbertubes navigation is because you have a couple people sucking bandwidth for Internet radio or videos or something like that then the proxy probably won't be a big boost.

One bonus to the proxy, though, is the ability to get a general idea of what kind of web traffic you're getting if the boss wants that information for some reason. For some businesses there are accountability issues, or you can see things like how hard update sites are getting hit (windows updates) or if there's anomalous traffic you will know what machine to investigate to see if it's infected with something and trying to "phone home".

The proxy would let you break down web traffic and get some statistics for your network in case there are some issues there.

Personally I don't see why you couldn't throw together a testbed proxy server and see if it helps or hurts your network. If it doesn't help, you just stop routing traffic through it and remove it from the network. If it does work, you could only benefit from the effort.

What type of internet connection do you have? What is the bandwidth speed? How many users do you have and what is their typical use of the internet? Are you currently having any bandwidth issues?

Read the first two paragraphs here and see if that helps.

Our ResTek group does something creative with their cache-proxy (which I believe is actually Squid) and our network infrastructure. Since they do not have unlimited bandwidth, they are using a packet-shaper to try and keep the P2P stuff prioritized lower. Traffic from the cache-proxy is prioritized highest. Regular ole web-traffic from dorm rooms is prioritized at a medium level.

The students have found out that they can get noticeably faster web access when they are configured to use the cache-proxy. This was intentional, as their networking people really would like to prioritize interactive web-traffic over bulk-downloads. By setting up the cache-proxy for high priority they do this.

Another potential benefit that has not been mentioned is for software upgrades. Instead of hitting the same upgrade site with many machines, if the updates/upgrades are cached, your internal machines will all upgrade much faster. These updates are generally static patch files so will be cached if squid is configured correctly. You can save a lot of traffic this way.