This is a single speed blower. Today it is not putting out the flow that it normally did; basically none. The blower still runs although, it sounds to me, not as fast as normal. I have taken the filters off and checked the ductwork. And the motor isn't making any unusual noises (such as a bad bearing would). Is it possible for the motor to run slower instead of stopping altogether, if it goes bad. (and how do I check this)?

A bad run capacitor will make the blower run slower. It's a cheap and easy part to replace. Take the capacitor off and have it tested at you local hvac supplier. If it test good, make sure you have good voltage. If all checks out good its time to buy a new motor and always buy a new capacitor for the motor you bought.