If no worms are seen
If concerned but not seeing any worms, a parent can get a lab order form for 'stool O and P' tests from his doctor. The lab will give the patient little plastic containers, to use on 3 separate days.
At home, the patient puts some stool into a container (regardless of whether or not he sees anything unusual in the stool). Then, the lab can process the stool sample to look for worms or worm eggs.