That feeling of “too good to be true" will nag you and make you feel like you are being paranoid. You are not paranoid. Sometimes too good to be true is because it's not true. Sure, sometimes good things happen to us that are unbelievable and we may even question wether we are worthy of such a perfect love, but that is different than the nagging feeling something might be off.
I have said it before and will say it again, narcissists have empathy. They are able to use it to know what you want and feel and how to blow your mind in the beginning. Then at some point they will start to use that empathy as a weapon against you.
-You feel that this person might be too good for you or out of your league and they don't discourage this feeling by empowering you.
-There are inconsistencies in their stories and actions even if their stories and actions seem to have an altruistic theme.
-They live beyond their means. A narc might convince themselves they deserve more than what they have earned. They will often have a pattern of obtaining this lifestyle without having to work for it.
-Conveniently unavailable whenever you want or need something that doesn't benefit the narc socially or financially. There are usually really good excuses.
-Narcs will go above and beyond in sexual encounters. Sometimes they will use patterns of hot an cold behavior to create an addiction like sexual relationship.
-Narcs usually won't take any responsibility for their previous relationships falling apart. It will sound like they had a string of bad luck, that they were victims and none of their own mistakes contributed to the end of the relationship. It isn't one or two, it's all of them, including family and friends.
-Narcs don't work in service oriented fields. This is something I have noticed. Unlike sociopaths who might place themselves in a service position to gain access to vulnerable people, narcs don't work in service fields because they consider themselves above that. They might work in social work or medicine though; a field that allows them to feel superior to others and have authority over their lives.
-Call them on a mistake early in the relationship. Doesn't need to be big, just point it out. See how they react. Can they admit they made a mistake without turning it on you?
-Listen for story telling inconsistencies. Does the story change based on who is listening? Do credentials and accomplishments change based on who is listening?
-Meet their friends and family sooner than later. Watch how the narc treats them, not necessarily how they treat the narc. I say that because lots of people have family that treat them like shit. Don't judge a person for strained family relationships, but DO look to see if their complaints about others are actually the way the narc treats THEM instead of the other way around. If the narc says their mom insults them a lot and judges them a lot and then when they are together it's the narc insulting and judging instead it's a giant red flag.
-Explosive anger when confronted with genuine consequences of bad decisions or actions. This is the guy who yells at the bank teller for charging them an overdraft fee that the guy totally deserved. This is the girl you are driving around that decides to stop at the pharmacy on the way to a job interview even though there is obviously not enough time. She gets to the interview late, doesn't get the job and yells at you about it while blaming the interviewer too.
-Disdain for authority. There are a million different reasonable explanations as to why anyone of authority can't be trusted or are employees of the people who should do the bidding of others. Disdain for government and having to follow societal rules. Some people are natural rebels who sometimes go against the grain, but are still law abiding and see the value in an organized system with consequences. Narcs resent authority and will complain about it endlessly, but will often DO nothing to promote positive social change. The system just doesn't apply to them so it doesn't matter.
-Religious or spiritual beliefs that allow them to feel innately superior to others for reasons that those people do and don't have control over. The narc is more aware, enlightened or more in God's grace than others.
-They are too much like you. They say what you would have said. They do what you would have done. They like what you like. They dress in a fashion almost just like yours. They have the same background as you. They have experienced all the same things you have. They tell stories of responding to things exactly like you would have. They like and dislike the same types of people you do. They have the exact same hopes and dreams. They see the world in the exact same way as you do. There are no areas where you clash. This is called mirroring, knowing what I know now, this would be enough to make me run.
THIS IS NOT SUSTAINABLE. Most people don't agree with you on everything. It’s more normal if there are areas they are blatantly different than you that you will have to decide if you can tolerate or not.
Some traits that are more common in people in general:
Talking over people
Turning stories other people tell into their stories
Using sympathy in place of empathy
Being generous only when it will be viewed by a group they aspire to be part of.
Talking their way out of fees, taxes and dues
Having acquaintances that serve a purpose rather than providing trust or intimacy
A lot of debt both financial and in favors
Comments that seem in support of people with dictator like qualities.
Blaming victims for their abuse and masking as being objective
Showing contempt or disgust for people who show vulnerability (weakness in the eyes of the narc).
Generally siding with the victor despite obvious moral questions.
**I could probably keep going but will stop there for now. Hope this helps.