This isn't a question about Tor. It really boils down to: How do I programmatically check whether two websites are the same? 
Where those two websites are, 1) the site when browsing without Tor, 2) the site when browsing with Tor. (Assuming you won't be comprimised in any way by browsing the site without using Tor.)
At the most simplistic level, you could just compare the contents of the Response objects returned by the requests API. To do a complete job would be a lot more work. You'd effectively have to crawl each of the two sites, following links to child pages that may or may not be in different sub-domains (and so may have different firewall/filtering profiles), and deal with situations where user interaction is required (which would need to be handled by your script).
I'm not one for pessimism, and I certainly value the problem-solving/fun aspects of such a project, but I'd be inclined to find a pre-existing tool that does the job for you. There are a few threads on the Stack Overflow site that might also be of help.
Finally, on the Tor wiki there's a list of services known to block Tor, together with a list of software that can be used to do the blocking. Perhaps working out a way to check for signatures of the blocking software would be another approach. (This might be more complicated that just checking for Captchas.)