How can you tell if your friend is jealous?

Me and my friend likes the same boy. And one today we were all huddled in a group just talking. Then the boy was around me and my friends. Then all of my friends said "there's your boyfrined!" to me. And my friend that likes the same boy as me just has her back against us and had a dirty look on her face. Then i told one of my friends in homeroom what look she had on her face and my friend said she was rolling her eyes and looked like a brat. But my friend said that she needs to get over it because she knows that he likes me. So how can i tell if she's jealouse?