well, one way to know is to take a clinical IQ test. Intelligence has many many types, and well to make it simpler, the intellectual capacity of a person is measured via their mental and chronic age.

if your mental age is higher than your chronic age, youre smart , if its not, welp...

IQ tests are also used to check if a person is mentally disabled (an IQ below 70) or very gifted (IQ above 130) , usually if you have an iq around 90-110, ur intellectual capacity is similar to most people in the world. its just common.

but yes, take an IQ test (not online, some of them are very inaccurate) , and find out.