Per saturn's comment, online tests can be pretty accurate, especially the ones which are imitating (copying) the matrix-style tests; I keep a list as part of the DNB FAQ.

Note the many caveats. In particular, you cannot take multiple tests! Obviously for most of them you can't take it twice because the questions don't change, but less obviously, they're all similar enough that if you take one, you can expect your score on the second to be noticeably increased just from familiarity/experience. (This is why I suggest that people doing dual n-back do before/after IQ tests with a minimum of months in between, and preferably years.)