I love to look for them and usually try to ask cathedral docents if there are any around. Sometimes they are visible, sometimes you need binoculars. The link to this piece on BBC popped up on my FB feed today and I thought others might find it interesting!
http://www.bbc.com/culture/story/20190104-the-surprising-roots-of-the-mysterious-green-man
Adding the one shown in Exeter Cathedral to my "to-do" list!