The following shell command tells you the branch that you are currently in.
git branch | grep ^\*
When you don't want to type that long command every time you want to know the branch and you are using Bash, give the command a short alias, for example alias cb, like so.
alias cb='git branch | grep ^\*'
When you are in branch master and your prompt is $, you will get * master as follows.
$ cb
* master