We are encouraging local people to take the lead in protecting the biodiversity of thier forests. I would like to gradually evaluate without any bias if our action have an impact on the ground
Well, you cannot ask directly. I just recently have read about your project in - you will not believe - Lithuanian language! So, it must be known also in other places? Then - do you have a sociologist in your team? If not, best idea perhaps is to have one (I do not mean employing, you may cooperate). Not only advice, but also participation of sociologist will save you time and efforts, you will not have to correct mistakes after questioning.
In my own opinion, your study's design has an evaluative approach - project or program evaluation. In this sense, you are trying to carry out a kind of formative and summative evaluation where you evaluate your participants when the project is on - going and after completion. So, I advice that you look at the general purpose and specific objectives for which you have set out your study. You may then adopt a questionnaire that goes in line with your set objectives. The questionnaire may be such that addresses some descriptive analysis like percentage distribution, this will help you to know how well you are moving by asking them simple questions based on your set objectives . Then you can use a simple statistical technique like correlated t-test to know the effectiveness of your intervention between BEFORE (collection of baseline information) and AFTER you have exposed them to various ways to protect bio-diversity of their forests. I believe this will help you a great deal.