Summarize:

Look in your basement or crawlspace for your sewer pipe. Follow the direction of the sewer pipe to identify the general location of your septic system. Go back outside and look around in that area to identify your septic tank.  Find a pipe that you know is a drain, such as a pipe coming from a toilet or a sink, and follow it until it connects to a larger pipe. The larger pipe is your sewer pipe. Once you locate your septic tank, draw a map of its location so you can find it in the future. If your tank is buried underground, you'll need to expose the top of it so you can inspect and access it. Use a shovel to remove enough dirt to allow you to see the top of the tank and the manhole.  Be careful not to drive the blade of the shovel into the septic system when you dig. Uncover the tank just enough to inspect it so you can bury it again when you're done so it isn't visible. Whenever you expose the top of the tank, look over the surface of it. Look for rust, dents, cracks, or any other signs of damage to the tank. Serious damage will require an inspection and possibly repairs from a septic tank specialist. Lots of corrosion and rust could mean it's time for you to replace your tank. A simple way to test if your plumbing is working correctly and traveling all the way to the septic tank is to check the connections by running water through them. Stand near the tank, have someone flush a toilet, and listen for the water traveling to the tank. If you see water bubbling through the ground or a crack in the system, your tank is in need of a repair from a septic tank specialist.
Follow your sewer pipe to locate your septic tank. Remove the dirt covering the top of your tank. Check for any cracks or damage to the septic tank. Flush a toilet to make sure the tank is working properly.