You can ask your affiliate manager/product owners to place your Google pixel code into their "Thank You" page. Anyone who is serious about their affiliates would be happy to place these codes for you. You'll be amazed how some networks are quick in responding such request. So don't be afraid to ask.
You can also track your Google Adwords campaign (keyword level) by using Subids (from the network) along with a tracking tool. With that, you'll be able to know how visitor got to your website and which keywords in your campaign are bringing in the sales.
You simply change your destination urls in your Google Adwords campaign to enable keyword tracking. Whenever someone clicks your ads, the {KeyWord} variable in your destination URL (the keyword visitor searched for) will be passed to your tracking url (outbound link to merchant page) which also contained your subid from the network.
So, When someone purchased a product through your link, you'll be able to know which keyword he/she searched for before getting to your site and how many visitors who reached your website through this keyword has actually bought something.
I've been using Prosper202 for months now and I'd recommend anyone to use it as well. It's free and easy to use. Just watch all videos and tutorials in the website and you'll be able to set up your very own tracking tool in your hosting account within an hour.
Prosper202
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http://prosper202.com/apps/