Hi Rio, you probably know that scientific inquiry actually is a very well researched area in science education.There are many assessment instruments for different types of students from schools to universities. It depends on your sample and your research questions which of the instruments you use, you should compare them based on their theoretical framework and the actual items. Some tend to have simply a quick look on Cronbach's Alpha for the reliability, this is certainly not enough - compare the actual wording of the items and whether or not they cover the aspects of inquiry that are important for your research (content validity).
If you are asking for a recommendation, this publication should serve you as an excellent starting point:
If your purpose is to measure students or teachers understandings of scientific inquiry, Christoph's recommendation is excellent. Of course, I am biased. However, it was not clear from your question that this is your intention. If you are interested in assessing the level of inquiry in an activity, then I would suggest the Herron Scale. It runs from 0-3 and assesses what information students have. If the students already know the question, the procedures, and the expected answer then the activity is a 0. If they are given a question and the procedures, but do not have any idea about the answer, it is a level 1. If all the students are given is the question, you have a level 2. Finally, if students are expected to develop the question, procedures, etc. it is a level 3.
I purpose you this entitled paper: The influence of explicit reflective versus implicit inquiry-oriented instruction on sixth graders’ views of nature of science. Khishfe and Abd-El-Khalick Journal of Research in Science Teaching, 39(7), 551-578.