How to measure web assets browser cache efficiency

Do You (smart guys :) have any idea what is the best way to measure cache efficieny of assets which are used on website (js, css, fonts etc).

How should I decide if for my website better way is (for example) to put all my JS files into one file or to seperate between some smaller files?

Many websites do it with one, two, three big files, but for example Facebook has many, many small files. It's clear that Facebook has a lot of returning visitors, so this strategy is better. But how to measure it.

I know I can check for GA and returning/new visitors ratio, but it's not very deep check. After that I still don't know which users during entering my website had some my assets in cache & which hadn't.