1. Decide if you think your partner feels superior. Since your partner always thinks you're wrong, there's a possibility that they may think they're superior. That is, if your partner feels superior to you in every way, they are more likely to to try to convince you that they're right and you're wrong.
Does your partner make statements that could indicate they feel superior? For instance, they might say (in seriousness, not jest), "Well, you know I'm smarter, so obviously I'm right."
2. Look to see if you are constantly changing to what they want. A narcissist literally thinks the world revolves around them. They expect to go to dinner where they want, watch what they want, and show up when they want without consequences. The problem is, they hold you to a different standard.
That is, a narcissist has no problem showing up very late (even an hour or more) without an apology. If you do that, you may find you're expected to apologize and never do it again.
3. Check to see if nothing meets their standards. Narcissists often have very high standards. Because they have trouble seeing outside themselves, they can't see how certain expectations may be too much. They also don't see all the hard work you have put into doing something. That's why it will likely seem like they always expect more than what you have to give. Also, they'll be much more likely to remember the things that haven't gone right than to remember the good things you've done.