If you have access to the attic; and presumably the top of the bathroom fan, you may be able to follow the duct.
If you don't have access to the top side of the fan, you could remove the cover and take a peek inside. You should be able to get a glimpse of the outlet, which should allow you to determine if there's ducting attached.
In my house, the bathroom exhaust fans vent directly into the insulation around them (an issue I've been meaning to fix since I moved in). Because of this I don't use the fans, since I'd rather have the moisture in the conditioned space than in the insulation.
If you've determined that there is actually ducting attached to the fan, you could use a fog machine to help locate the outlet.
You can usually pick these up fairly cheap; especially around Halloween, or possibly rent one from a party supply store. Set the machine up in the bathroom, and turn on the exhaust fan. Then walk around your house looking for the fog.
NOTE: You might want to set the fog machine up on a ladder, as close to the exhaust fan as possible. Otherwise you might have to fill the entire bathroom with fog, before it's actually sucked into the exhaust fan. The fog tends to be heavier than air, and so tends to hang around the floor.
Smoke pellets, smoke candles, and other smoke testing devices are also available. Which may work better than a fog machine, since the smoke from these are typically lighter than air.
The product links are for demonstration purposes only, I have never used these specific products and do not endorse nor recommend these specific products.
