In the absense of writing experience in my target field, how do I find assignments and evaluate them?
The obvious way to get experience along with feedback is to enroll in a formal writing course. But try to find one in which the instructor is amenable to you making suggestions about general areas and topics. But any writing will be better than no writing. 

A second, but riskier way, is to publish a blog somewhere in which you express your thoughts but allow readers to comment and give you feedback. The world is full of trolls, of course, so be prepared for that, including a way to shut them off and/or back out of the whole project. 

Even writing here on this site you get some feedback in the comments.