Most Helpful Girl
When they don't make time for you. A big thing too is if they only talk to you like at night when they're bored, that means that they had nothing better to do and wanted some entertainment. If they really like you, they'll talk to you during the day too, they'll make time for you. Starting a conversation with, "I'm bored" is a bad sign too.
Also, if a person doesn't ask you questions about yourself, that's not a good sign either. Most the time that means that they aren't actually interested in you or they just don't really give a shit5
Most Helpful Guy
You can tell when she is constantly changing. If she’s all in to you one day then the next she’s basically ignoring you that’s a bad sign if it keeps happening. If she keeps mentioning a guy or if she keeps making lame excuses to keep from seeing you.3