Most of the time the "test", if it may really be called as such, is that some enterprise (=large business), has deployed a successful and stable product using it. So its more like saying its proven its worth on the battlefield, or something like that. In other words the framework has been used successfully, or not in the real world, you can't just follow some checklist and load tests and say its enterprise ready.
To be short:
"Enterprise ready" means: If it crashes, the enterprises using it will possibly sue you.
From my experience, "Enterprise ready" label is an indicator of the fear of managers to adopt an open-source technology, possibly balanced with a desire not to stay follower in that technology.
This may objectively argued with considerations such as support from a third party company or integration in existing development tools.
Like Robert Gould says in his answer, it's "Enterprise-ready" when it's been proven by some other huge project. I'd put it this way: if somebody out there has made millions of dollars with it and gotten written up by venture capitalist magazines as the year's (some year, not necessarily this one) hottest new thing, then it's Enterprise-ready. :)
Another way to look at the question is that a tech is Enterprise-ready when a non-tech boss or business owner won't worry about whether or not they've chosen a good platform to run their business on. In this sense Enterprise-ready is a measure of brand recognition rather than technological maturity.
I suppose an application could be considered "enterprise ready" when it is stable enough that a large company would use it. It would also imply some level of support, so when it does inevitable break.
Wether or not something is "enterprise ready" is entirely subjective, and undefined, and rather "buzz word'y".. Basically, you can't have a test_isEnterpriseReady() - just make your application as reliable and efficient as it can be..
"Enterprise Ready" for the most part means can we run it raliably and effectivly within a large organisation.
There are several factors involved:
Depending on how important the system is to the business the follwiing question might also apply:
Open Source projects often do not pay enough attention to the difficulties of deploying and running software within a large organisation. e.g. Most OS projects default to MySql as the database, which is a good and sensible choice for most small projects, however, if you Enterprise has an ORACLE site license and a team of highly skilled ORACLE DBAs in place the MySql option looks distinctly unattractive.
Having built a couple "Enterprise" applications...
Enterprise outside of development means, that if it breaks, someone can fix it. I've worked with employers/contractors that stick with quite possibly the worst managing hosting providers, data vendors, or such because they will fix problems when they crop up, even if they crop up a lot it, and have someone to call when they break.
So to restate it another way, Enterprise software is Enterprisey because it has support options available. A simple example: jQuery isn't enterprisey while ExtJS is, because ExtJS has a corporate support structure to it. (Yes I know these two frameworks is like comparing a toolset to a factory manufactured home kit ).
As my day job is all about enterprise architecture, I believe that the word enterprise isn't nowadays about size nor scale but refers more to how a software product is sold.
For example, Ruby on Rails isn't enterprise because there is no vendor that will come into your shop and do Powerpoint presentations repeatedly for the developer community. Ruby on Rails doesn't have a sales executive that takes me out to the golf course or my favorite restaurant for lunch. Ruby on Rails also isn't deeply covered by industry analyst firms such as Gartner.
Ruby on Rails will never be considered "enterprise" until these things occur...