What images published in arXiv papers can I include in my education material? I am going to give a course at my company and I'm looking online for images that I can use in my education material.

Since many of the most interesting images suitable for this education exist in papers published on arXiv, how do I know which of the images found in arXiv papers I can use and which I can't? How do I know how these images are licenced?

I often see images published in arXiv papers that are later used in presentations given by other people and/or republished in other's research papers, sometimes without any credit given to the creators of the images and almost always without a link to any license (in fact I can't remember ever seeing such a link, but maybe I have just not paid attention). Under what circumstances is that okay?

A related question is, how does fair use come into all of this since we're talking about education material?
Contrary to the other answer, arXiv does not require that papers are posted under CC licences. The minimal (and normal) licence required is given here, and merely grants arXiv perpetual right to distribute the article. IANAL but I very much doubt this gives you any right to use individual figures. 

You can find a (small) link to the licence for a particular preprint on the right-hand side of the abstract page in arXiv. You should see something like



If there is a CC licence you can use the image but probably need to attribute. Otherwise, I would contact the authors to ask for permission.