A cusp is the moment one sign changes to the next. The cusp between Capricorn and Aquarius happens at a precise moment, which varies from year to year. If you were born before it you would be Capricorn, and after it you would be Aquarius.
The moment of the Capricorn Aquarius cusp is not related to calendar dates. It's not as simple as saying January 18th is always Capricorn and January 19th is always Aquarius. One year the cusp may be at 11pm at night on January 18th, and the next year it may be early in the morning on January 19th etc.
To find out for sure which sign you are you will need to know when exactly you were born. Specifically you will need to know:
With that information an astrologer can calculate which side of the Capricorn Aquarius cusp you were born on, and therefore which sign you are. For example, this calculation is always performed as part of my natal readings (for one person) and relationship readings (for couples).
If you don't know your birth time then things get a little more complicated, but it is often possible to figure it out "backwards" from major life events. That's a technique called birth time rectification. I won't confuse things here by going into it, other than to say it exists for that scenario.
Another way to establish which sign you are is simply to look at the list of Capricorn traits compared to Aquarius traits. Capricorn and Aquarius have very different characteristics so it's often fairly easy to see which one you are.
In some cases however people relate almost equally to both signs. In that situation it's going to be because your sun sign is one of them, while you have another planet (most often mercury) in the other. For example you may have your sun in Capricorn and your mercury in Aquarius.
In that situation you probably will need an astrologer to tell you which is which, and explain how they interact. There are a large number of permutations so trying to cover them all here would be very confusing, but I automatically include your specific planets and interactions in all my natal readings (for one person) and relationship readings (for couples) mentioned above.
Find out what astrology has to say about your unique partner and relationship beyond just sun signs.
How to know if you're with your soulmate. The most common features of soulmate relationships.
By understanding someone and how they view things it's easy to make yourself more attractive to them.
Explore hidden factors such as physical attraction so you can see the whole picture of your relationship.
Personal relationship advice based on your natal and composite charts. 21 years of experience solving real problems for real couples.
Astrology can let you learn more about someone you recently met. It's the most powerful tool I've found to predict the future of a relationship.
Avoid arguments before they start and turn your differences into strengths. It's never too late to begin again.
For the first time in years you feel alive. My unbiased and anonymous analysis takes the gamble out of deciding what to do.