How do you know if you have a 'accident waiting to happen' going on in your heart or heart valves?

As far as i know, usually it's deposits of plaque which become fused to your arterial wall and hang around there for years, then break off during exertion, causing a blood clot, which can be the ticking time bomb . . . how do i check if i have one of these? I am 30 years old and about 50 pounds overweight, and eat a lot of junk food, not much salad.

Thoughts?