My experience with romantic relationships is very narrow. Those red flags I will mention are from my own experience.
You only have a month dating that person, and there are already signs of jealousy. For instance, she/he doesn't like when you talk to other people from the opposite sex.
By questioning that behaviour you may get answers like: "is because I care for you",
"jealousy is not that bad, been jealous actually means you care for someone".
After you become official it may not be evident, and you will possibly rationalize the contradictions of her/his words with her/his actions.
She/He will say things, without really meaning it. Things like a promise, or an initiative.
Her/His words are gonna sound serious, but her actions are not gonna aloud you to take them seriously, however you will surely become confused.
🚩NO ONE LIKES ME
This red flag is among the most evident red flags and one that is going to appear later on. If it includes her/his family, reconsider her/his mental state, and the future of your mental health.
Remember you are not superman and that her/his issues shouldn't be yours to some extent. Support and encouragement are all you can give in this case. Whatever issues she has with the world, she should solve it with the assistance of a mental health specialist.
🚩MOVING ON TOO FAST
It has to do more with the first red flag I mention. When someone wants to be your SO in a rush, it can mean 2 things:
1) Mental problems.
2) No experience in dating.
If your relationship is online, you will never know for sure, until you meet in person. Be careful and don't rush it.
🚩A POOR RECIPROCITY
Don't get me wrong, a person who is not able to be reciprocal in a romantic relationship is able to love. Been so, understand that whatever is keeping that person back, don't have anything to do with you. It all has to do with fear and toxic experiences. Some fears and experiences you will know, others she will keep as a secret, and you may never know them.
Now that pointed out and explained the 5 red flags, we should all consider this quote:
Infuse your life with action, don't wait for it to happen. Make it happen (...)
Every one of us has the right to be happy, but we also have the obligation to find happiness within our self.