This is a touchy subject for me.
Usually when I'm out and about I don't see many trans women just because there aren't many in my area. Yes, there have been a times I was able to notice another girl is trans (I hate the term "clocking," or "clocked" and refuse to use it). For me the things that usually give someone away are manurisms ie: the way they walk, their body language, etc., and especially nervousness only because I know what to look for, as others have pointed out.
There have been a few times where I was out with my girlfriends some knowing my history, some who didn't, that I've been very disappointed and in some instances even appalled by my friends attitudes toward transwomen who weren't as tiny and feminine built as those of us who are more fortunate. During these few times my girlfriends I was with would notice someone is trans by obvious physical attributes. They either had a large head, a high hairline, big hands and feet, they're overly tall, a prominent adams apple, deep brow, or noticeable beard shadow, but mostly features they couldn't help. Because I'm lucky enough to not have these problems, they would start talking about them and how they look in front of me, expecting me to agree with them and join in the conversation. I was shocked my so called friends would behave this way in front of me. They tell me I'm different because of how I look. Of course I beg to differ and told them how ashamed I was to be seen with them. On several occasions when this happened I was horrified and left their group because of how they acted. And as a result of these encounters I've lost a lot of friends. I just have no use for anyone who takes pleasure in belittling others trans or not because of physical attributes they can't help.
I also believe as others have pointed out; It is easier for us to spot the little things in other trans people because we as trans ourselves know what to look for. The average cis public doesn't though, and generally aren't paying attention.
However, at a trans group meeting up north I've even had one other trans woman become angry with me during a conversation she brought up about passing. She looked at me kind of funny and stared saying: "well we can't all be 5-5 and 120 pounds." She was one of those who would out herself by constantly putting herself and her looks down. Actually I thought she was quite pretty.
Ali