The vet usually floats the fecal in a supersaturated salt solution that causes worm eggs to float. They put a slide on top of that and the eggs stick to the slide for them to look at them under a microscope. Tapeworm eggs do not show up this way, they are almost always found due to owner reporting. There are a few things that the vet can find from looking at some stool applied directly to a slide rather than floating, but it often depends on several factors whether or not they will find those things even when they are present. The lab is better equipped to find those things, usually.
