When I first heard of creativity tests I was surprised that this should be possible but I was quickly convinced that at least some aspects can indeed be tested relatively straitforward:

1) Originality - statistical infrequency of response 2) Fluency - number of responses 3) Flexibility - the degree of difference of the responses, in other words do they come from a single domain or multiple domains 4) Elaboration - the amount of detail of the response

(these are from the web here but recurr in standard tests like William’s Creativity Assessment Packet (CAP) and real scoring instructions that I have seen are interestingly simple to do. More interesting are actual examples provided which show the range of creativity. And that is where I think one can actually learn to be creative: By reproducing certain dimensions of creativity. That may sound like a contradiction in terms but I have found from self-practice that many creative ideas build on earlier ones. And the earlier ones apparently don't neccessarily need to be your own :-)

While looking for accessible material online I found these: A marketing campaign which stimulates creativity and a thesis which adapts the CAP to an online setting and may give you a feel of creativity tests in practice.