Identifying web platforms

I've recently realised that over the last couple of years, I've kept a much closer eye on what platforms websites are running. It's interesting to see parts of a site running on a different platform than the rest, or watching as a "redesign" suddenly comes with more than just cosmetic changes.

While there's a few ways to identify Web platforms, some are way more fun than others. Sure, sometimes looking for that "Power by Foo" logo is the easiest way, but it's not really satisfying.

More satisfying are things like:

Login box identification. The site design may be completely different to the stock standard installation, but somehow the login box has survived unscathed.

Registration process. This is the little brother to the login box identification. Most web platforms have standard registration processes for users which no one seems to really modify.

Standard URL. This is also more useful for identifying web platforms which don't provide some of the above, and is one of the most powerful methods. This technique relies on more than just identifying the actual URLs that are chosen but also the pattern that they take (and the Mod Rewrite rules that always seem to sneak out). More advanced techniques involve looking for unique identifiers, their pattern and where they occur

Form handling. Most platforms have specific ways in which they deal with forms, and specially with how they deal with error handling. Some messages can be so typical that while you may not be able to identify a platform, you'll recognise it straight away as the same platform that some other site is running.

Error messages. This is often one of the last resorts methods as you'll only see it when something is actually going wrong on the site. How a platform choses to react to its DB engine disappearing will often reveal exactly what it's running, or at least break in predictable ways. If you don't want to wait until the site fails however, you can get Google to help you by searching for some common error messages and the name of the site.

Generated code. Most platforms will generate code in a standard way. If you've used a platform a fair bit, you'll start recognising patterns emerging from the generated code. Some of them will be quite simple (for example, the way in which CSS files have been generated and inserted). Others will be a little harder (for example the naming of automatically generated divs).

So what's your favourite method of identifying which platform is being used to run a site?