When talking about a camera, for example the Canon 5D Mark II, plenty of people will state that one of its weaknesses is the "slow" auto focus. What is this compared to? Is there a standardization, or some type of reference?
There's no standard, or even agreement. Generally, people mean "compared to other cameras of that general class". Or maybe "compared to a sense of all cameras available now" — it depends on the context.
The answer may even change over time — a camera may be fast compared to the competition when it comes out, but a few years later be slow compared to the new generation. And, by itself, this is missing another crucial element: slow at what? Bad at tracking moving subjects? Slow to acquire focus lock? Hunts around in low light?
There's also no sense of scale without any context — does "slow" mean "excruciating to use" or does it mean "undetectable without measuring, but measurements show...."? Reviews often get caught up in the latter, because the Internet selects for reviews which seem serious because they measure things and provide numbers (as opposed to objective, in the field use), and the secret is that all cameras made today are extremely powerful and awesome, so there's incentive to make a big deal of small things. This is compounded by the disease imusthavethebeststuffitis, which leads to a the hobby of comparing and contrasting such things on forums — which, sure, can be a fine hobby, but ultimately less interesting than actually going out and making photographs.
Basically: you need context; "slow" doesn't mean much by itself.
It might be worth reading some of the LensRentals.com blog posts on Autofocus to understand some of the things they test and how they test them to determine AF speed. Most of the topic is testing accuracy, but they do cover speed to a degree.
How do you determine if a camera has fast or slow auto focus?
You use it and compare it to other cameras you have used in the same types of shooting situations. Do you get more keepers or fewer in rapidly changing situations where focusing speed matters? Do you miss shots with one camera while waiting for the AF that you could have gotten with the other camera?
Fast and Slow are always relative terms. How high does a car's top speed have to be to be considered a "fast car?" How restricted does a car's top speed need to be to be considered "slow?" Does how long it takes that car to reach its top speed have any bearing on whether it is considered fast or slow?
All of that depends on whom you ask and how they define "fast" and "slow." A drag racer bases their definition of "fast" on how long it takes to go 1/4 mile in a straight line from a standing start. An F1 racer defines "fast" as a combination of acceleration, top speed, and the ability to nimbly negotiate curves to complete a circuit from a running start. What the drag racer considers "fast" wouldn't be competitive on an F1 course. What the F1 racer considers "fast" wouldn't be competitive with top fuel cars on a 1/4 mile.
In the case of cameras and autofocus system performance there are just too many variables and different use cases to standardize a way of measuring one AF system against another, especially in terms of real world usage. This is the case, even with entry level and mid-grade AF systems that are not very configurable by the user.
To further compound the problem of standardizing AF performance metrics, the most sophisticated pro-level AF systems on the market today, such as those contained in the Canon EOS 1D X Mark II and Nikon D5, are highly configurable by the user to perform the way each user needs it too. The various combinations of AF settings with these cameras are almost endless and allow each user to customize the way the AF system acts to suit the situation in which they find themselves shooting. So even if one could come up with a "standard definition" of "fast" with regard to AF systems, it would still be difficult to rate each system using every possible permutation of the AF settings.
So the definition of "fast" often hinges on exactly what aspect of the AF system one needs to be fast to get the shot. But sometimes the definition of "slow" includes "all of the above."
In the case of the EOS 5D Mark II the AF system is more than just a bit slow regardless of how one defines "slow" within the context of "pro grade" DSLRs introduced in the last decade or so.
My 5D Mark II has more shutter actuations than any other camera I've ever owned. It's a great camera and is useful for many things and gives excellent images when used properly. But fast AF is not one of its strengths. It is slower to focus in bright light than the other non-Rebel EOS bodies I have used extensively: 50D (but not by much), 7D, 7D Mark II, 5D Mark III. It was slow compared to the other Canon "pro" bodies when it was introduced (1Ds Mark III, 1D Mark III). It is slower to track moving subjects. It is slower to focus in low light, if it can focus at all. It is slower with lenses that focus very fast on other EOS bodies. It is even slower to focus with lenses that focus less quickly on other bodies. And it's not like we get any increased accuracy or shot-to-shot consistency for all of that waiting on the AF system. Anyone who has ever shot with it realizes it's slower to AF than most other cameras in its class.
In the end the definition for "fast" or "slow" AF depends on the user. Does it allow you to get shots you couldn't get with the other cameras to which you are comparing it? Does it restrict your ability to take the shots you want? Do other cameras AF fast enough to get those same shots?