What's in Your Attic?

To start, you can listen. Animals make noise; they don't think to be sneaky. They aren't trying to be a ninja assassin. Some will make a lot. It's probably how you knew you had something living in your attic to begin with.
Are the noises being heard during the day or at night?

Squirrels

Active during the day. Flying squirrels are the only exception and are active at night. For all squirrels you'll hear a quick scurrying or acorns rolling around or chewing. If you hear chewing, you best get to getting them out fast. Squirrels are notorious for starting house fires by chewing wires. Danger, Will Robinson, danger.
Fact: Squirrels teeth are constantly growing. That's why they chew on anything and everything. It's to “file” them down and keep them at a suitable length.

Bats

Tend to make noises during sunset. They are leaving for the evening, so you'll hear a light scurrying along with high-pitched chirps.

Mice

Active during the nighttime hours, so you'll hear a fast pitter-patter.

Raccoons

You may hear sounds during the day, but you'll mostly hear their heavy thumping with calls and growls during the night.

Listening is only the first step and I've only mentioned the pests that predominately take up residence in attics. Start here by choosing a repellent that is specific to the animal you heard.

And another thing (that I know you may not want to hear) is that you really need to take a look or find a willing victim–er, helper–to do so.

Squirrels

Leave debris (other than their droppings) in the attic, such as acorns, leaves and twigs.

Bats and Mice

Use your attic as their personal bathroom, so that's easy enough to see.

Others

They're the loud ones, so you should be able to tell by listening.

You can also look for them to enter or leave around dawn and dusk.