How do I know if my childs sinus venosus asd will close by itself?

It won't. If relevant (based on size of the right heart) your child needs surgery after the first year of life. In rare cases we have succeeded in closing sinus venosus defects in the cath lab if there are no associated pulmonary vein anomalies.
Cardiologist. You will need to follow up as rec by your dr for repeat echocardiograms. Add is often silent on exam so echo is needed.