You could use one or more sub tests of Torrance Tests of Creativity Thinking. You can choose according to the type of creativity process you want to study.

Another way, more indirect but interesting, is to assign your own task and record psycho-phisiological indexes using, for example, a biofeedback equipment.

Feel free to write me if you need more information.

I am aware of Torrance Tests of Creativity . I am with the impression that it is measuring generally how creative a person is, rather than "in the moment" creativity?

I believe it also reflects "in the moment" creativity because it asks individuals to perform a creative task (verbal or visual) and measures his/her performance, considering different factors, on that specific task. If you add timing and response order as variables to your final statistic model, you will probably be able to have a good reliable measure.

But I am still thinking about your answer and trying to find other suitable suggestions :-)

as in "Flow"? Look at the work of Mihaly Csikszentmihalyi and his lab

I believe that such a test is possible, but, not practical.

My research lends support to the idea that life is a fundamental physical process and learning and creativity are the consequence of a basic economy.

Imagine a child learning to play the piano. The teacher says that the child must learn the left hand part and the right hand part and then they must come together. Many have had the painful experience of listening to some play when the left and the right sides of things have not quite come together.

Intuitively, we feel that the moment that the child attains a 'higher synthesis" that there must be a corresponding increase in the complexity of what is happening in the brain. On the contrary, fMRI studies suggest that after the synthesis takes place the resulting brain workload is LESS than it was before.

The measure of the creative moment would be a quantity derived from the difference between the energy/work expended on a task before the moment of creativity compared with the energy/work expended on the task after the moment of creativity.

Actually, measure" in the moment" creativity is an abstract idea. It seems cannot be standardize as a measurement in population. If this is the case, self evaluation will help.

Try to use this items:

- I am a creative person

- I always can create a novel and useful solution

- I always facilitate myself to be a creative person

- I am well understand the definition of creativity

...etc

If in small class evaluation...5 pt scale is good enough.

I don't see the Torrance Tests, or other measures of Divergent Thinking, as "in the moment." Csikszentmihalyi looked at something similar by giving beepers to people, and they were beeped at random times and shared what was on their minds. But boy, "in the moment" may be close to impossible for the same reason that introspection is not a perfect method: If you are introspecting, you are not being spontaneous, so what you "observe" is not indicative of anything other than how you think when you are introspecting. The neuroscientific measures would be best, but then you get chemical and structural information, which is not easily tied to thinking (or consciousness). Tough question! I would like to hear more about the objective behind this question.

Maybe have a look in my Handbook o Management Scales: https://en.wikibooks.org/wiki/Handbook_of_Management_Scales I could not find a scale that you are looking for, but maybe you will find a related scale there?

I am still thinking of an In The Moment measure of Creativity. I really like the idea so went through all 30 plus tests available at www.creativitytestingservices.com    I was trying to get ideas for in the moment. The spontaneity of in the moment may preclude measurement. But then again, Csikszentmihalyi did get at flow, so maybe.... Thank you for asking a provocative question. Food for thought. 

Do you mean an operationalization of Flow? Interesting.  I think Csikszentmihalyi had an interesting design for getting at Flow. Did he not adapt a method of using random calls / reminders to have participants record their thoughts etc. in the moment?