Is it acceptable to route a bathroom exhaust fan to the return of a central air system? I'm afraid this will cause mold to grow in the air return ducts. There is currently a fan in our bathroom and I can't seem to find any separate outlet or vent for it, so I assume that it must be connected our central air system somehow. What is the best practice for routing bathroom fans if mold is a concern?
A bathroom exhaust fan should vent directly outside with its own dedicated duct, for exactly the reasons you are stating.
I have seen where a bathroom fan is run to a charcoal filter and then vented back into the interior of the house. A terrible idea, as you noted, it is a recipe for mold!
While it may be acceptable to vent into the central heater during winter when humidity is low, it is a terrible idea year round. The humidity load of the bathroom vent air is just too high. You want an external vent. And you very much want to trace that vent... if it goes to the attic that's quite bad.