So, what is the pixel? It's a handy dandy little code unique to your ads manager that allows you to capture information from users who visit your website, and then re-target them on Facebook. It also can provide conversion data like when someone fills out a form, makes a purchase, and so forth. It's crucial for your re-targeting efforts AND for tracking purposes, to get that pixel working!
Now first things first, is the pixel even installed on the site? For that I recommend just going to the pixel section of your ads manager and right away you'll see data showing visits if it's working, or you'll see instructions to install it if it's not.
|Pixel not installed yet|
|Pixel installed and tracking|
You can also try using the pixel helper Chrome extension. Trust me, it's easy, so I'm not doing a tutorial on that.
But that doesn't really tell the whole story...The quick way I check to see if a pixel is working, is to navigate to the pixel area again, but look down at the actual events listed.
Then check to see if the event codes you installed are firing. If all you installed was the base code, you'll most likely just see "page view" and a number. (See photo below)
If you're trying to track other actions on your site, you'll see those listed too. One of the most common issues I see is every action/event has the same number. If every event has the same number, you have not installed the events correctly. In general, you should see something that tapers down. A large amount of page views, a lesser amount of add to carts, and an even smaller amount of purchases for example. Same thing for leads, registrations, or whatever event you're really looking to track.
You will also get data about what time the last action occurred, so this should match what you see on your website analytics.
I hope this helps!