What would be the first thing a mechanic would check if a motor was running rough?

My van was running rough, and the dealership where I purchased the vehicle took it to
his mechanic to have it repaired. He promised me it was taken care of, but when I
drove it after-ward it was still running rough. I took it to my mechanic and he said that
there were carbon marks on the spark plugs that might have been causing the problem
and changed the plugs and wires. I am just wondering if this would be the first thing
that a mechanic would check when looking at a engine problem like this.