Alternate Methods Of Getting Last URL

Using HTTP_REFERER I cannot capture the URL when the link is made via a
button:

<input type="button" name="Continue" value="Continue Shopping"
onclick="ContinueShopping('http://mysite/supertest.asp');">

I do not control the script above, it is on an e-commerce site that is very
generic, meant for many users - and is generated by their server code. I
want to capture the URL and with an if/then redirect if the last place
visited was the checkout site.

I need an alternate method of URL capture, rather than HTTP_REFERER.