compression ratio

i am putting in my turbo later...
but coz my car is a 93... and it has around 90000 miles on it already.. i am afraid about the engine's condition....... this guy has all the tune ups and maintainece done at a regular period.. so i think he did keep it well...
but will a compression ratio test whta i need to know???

and what exactly is a compression ratio???

let me know

thanks