If you have a septic system that uses a grease trap, keep a close eye on the trap to ensure it doesn't clog up. You don't have to dive into the system yourself to find the trap, though; a clog usually gives off telltale warning signs. Because a clog anywhere in your septic system is bad news, if you see -- or smell -- any of these signs, have a company come out to clean the trap. If the trap is in bad shape or you notice it clogging up frequently despite your best efforts, you may have to have it changed.
Any grease or other solids clogging up the trap will smell, and they'll smell rather strongly. If you notice a smell coming out of one drain in your home, try turning on the water for a few minutes, turning off the water, and then seeing if the smell goes away after a day or so. If it does, the problem was likely that the water plug in the pipe leading away from the drain had dried up. But if the smell doesn't go away, or you start noticing it from multiple faucets, that is a sign of a septic clog.
If the grease trap is clogged, nothing will be able to get through it. Even water will not get through, causing backups into sinks when you turn on the water. Slow drains can be a tricky symptom because so many things can lead to them. First, if the drains are slow and smell terrible, and you haven't put anything odd down the drain, that's a sign you need to call septic contractors. If there's no smell, and the drain is slow or clogged, then you should try the vinegar-baking soda cleaner combination (pour some baking soda into the drain, followed by white vinegar; cover the drain so that none of the mixture bubbles out, and then pour hot water down the drain, repeating the whole procedure a couple of times). However, if that doesn't work, call a plumber who is familiar with septic systems, or a septic contractor who also works on plumbing.
If you've moved into a house that's new to you (so the house itself isn't new), you may want to have the grease trap totally overhauled if you're not sure what condition it's in. It's best to start off with a septic system that's in the best shape possible. Talk to septic contractors, like Moon Site & Septic, about inspecting the whole system and cleaning all of it out so that you have a literal fresh start. That will help you keep the whole property in better shape, too.