i have a 2002 vette how do i tell how many horsepower it has

Standard 2002 Corvette has 350 HP, if it is a Z06 it has 405 HP

For any actual read, go to a dyne shop for a read. What is advertised is what engineers on paper say it should be. They usually are close. Depending on a lot of outside factors will make the HP readings go up or down. Most dyno shops will also offer a tune-up to go with the readings to get what you desire as far as output. Top HP or economy or middle ground. Each car coming off the assembly line will read different, but are close to post the average.