I've seen several children attempt to throw cognitive assessments (and these are, of course, only the ones that I caught at it). Almost always it has been because they believe the outcome of doing well on the test will be something undesirable (although fatigue or impatience result in a low estimate, too, I don't really think of that as deliberate sabotage), such as, having to leave their neighborhood school and its community for another "special" program, being placed in more challenging classes, having to leave a sheltered support program. Once, I had a student deliberately underperform in an attempt to get -into- a special education setting.
My experience has been that kids are better at faking bad on the knowledge-based parts of tests, and often get tripped up (so to speak) on the nonverbal portions, which are less academic in appearance. And, yes, usually one can tell by the inconsistencies in performance within and between subtests. Again, this only applies to the kids I've caught.
This is why I like to spend a little extra time up front discussing the purpose of testing with children of all ages. I generally begin by asking them if anyone has talked to them about it, and what they think its purpose is. To that last point, I try to emphasize its value for self-knowledge, not so much for rank-ordering or placement decisions. For some children, I also point out that this kind of testing is only done once, or every few years, so it's important that we get a true picture of them.
