One way of finding how smart you are is by taking an IQ test. These tests measure aptitude in different cognitive areas, giving you a number that relates to where you are on a scale.
There are more ways to be smart than simple intelligence potential. Wisdom gained from experiences can bestow empirical knowledge from experiences. This can allow you to have "street smarts." Keeping up with current affairs and being well read also increases your intelligence. While IQ tests are good indicators, they should not be the only thing used to gauge intelligence, since humans have different capacities to learn spatially, mathematically and verbally.