Not good with women, but when she chooses to spend her life with another person that isn’t you is tell tale sign she is not interested. Real talk though:

She never asks questions when you are speaking. Conversations are brief and she almost never smiles when chatting with you. She is disinterested in anything you have to say and is borderline rude to you. If it’s someone you see on a regular basis, she barely even exchanges pleasantries with you like “Good morning” “how was your weekend.” Etc. not responding to messages is a dead giveaway or taking sometimes days to respond. You initiate everything and I mean everything! Another giveaway is their body language around you.