You can "look it up" from online results sites like OPR and SharkScope. Or if you run a tracker it will keep up with it for you. Or if you're old-school you can calculate it yourself if you keep up with your buy-ins and results. The latter two are going to be more accurate since the results sites rarely get every tournament/SNG correct.
Beware these sites are not 100% accurate. My last 5 months at Full Tilt never showed up. Funny, because thats when I became profitable.