My brother suggests continuous performance tasks.
This is a review of four CPT packages from 2000, and this is an review of 3 CPT packages from 1995 from a user's perspective. These are some slides from 2003 about computerized CPTs and related test instruments.
Continuous performance tasks test sustained attention and need to be "purposefully boring", typically 10-25 minutes, which can be too long for daily use.
The Psychology Experiment Building Language comes with a battery of tests "designed to closely resemble many commonly-used psychological tests", including clones of the Test of Variables of Attention, Conners' Continuous Performance Task, the Wisconsin Card Sorting Task, an implicit association test, and the Iowa Gambling Task.
Some free platforms that could be used to build tests are psychopy, the Vision Egg, pyepl, and PEBL. Some other platforms are in this list.
Any psychological test of response time that uses only consumer PC hardware will have difficulty with timing resolution and calibration. Platforms like operating systems, libraries, and virtual machines, and competition by other processes for processor time, can add difficulty. Some operating systems have a scheduler resolution like 10ms on an unloaded system, and they put input events into a queue without recording when the events happened. Even if an operating system does not do this, a cross-platform media I/O platform like Flash (used by Cognitive Fun,...
We all have our good days and our bad days. Due to insufficient sleep, illness, stress, distractions, and many other causes we often find ourselves far below our usual levels of mental performance. When we find ourselves in such a state, it's not really worth putting effort in doing many tasks, like programming or long term planning - as quality will suffer a lot.
The problem is - other than observing deterioration of results, I have no idea if I'm in such a state or not. I cannot be sure if it's also true for others, but I had to find out a few tests of what's my mental performance at the moment. Tests that are deeply flawed, so I'd request better if there are any. I also cannot predict my mental state in advance, as my life isn't terribly regular.
The most reliable test I found, and by accident, was fighting bots on a certain Quake 3 map - me vs 10 or so highest difficulty bots. The challenge was to get 50 frags without dying. As the map was huge and full of power ups, it wasn't really that difficult as long as I could maintain full alertness for 10-15 minutes - but if I was tired or distracted, I would invariably fail. This test was unfortunately extremely slow.
Another test would be to go to goproblems, and do a few random problems at proper difficulty level. If I could think right, I would do most of them, if I was tired, I would fail almost 100%. This didn't test alertness, I guess it would be best described as short term memory test, as that's what used for game tree exploration. Unfortunately what's the proper difficulty varies a lot with how much go I played recently, so it needs to be recalibrated.
One more test would be to go to some decent online IQ test like this one. My results on such test would suffer a lot if I was sleepy or tired. The main problem is that such tests cannot repeated too often, or I'd just remember the answers.
So these are three ways to test how well my mind functions at the moment, all testing something different, and all flawed in one way or another.
How do you test yourself?