Just look at their policy recommendations. It’s easy to tell from that. As far as I know, they are not usually funded by parties. They generally raise funds directly from supporters. Some apply for and receive government grants. For others, you have to look at their client lists. At the think tank I used to work for, our clients included unions, various government entities, a medical society, several different foundations that were interested in a wide variety of issues from health care to immigration reform. Our mission statement included the word “progressive,” I believe.