If your plant looks sick and you're not under- or overwatering, check in the folds of leaves, the joints where leaves meet stem and on the undersides of leaves. If you see any creepy crawlers, move on to the next step. If you don't see any pests but do see a sticky substance on the leaves, this is called honeydew, excretion from pests that are feeding on your plant. Pull the plant up and out of its pot to examine the rootball, as pests could be hiding there too.
The most common plant pests you're likely to encounter in a Midwestern home or business are spider mites, mealybugs, aphids, whiteflies, fungus gnats and scale, so familiarize yourself with how to identify those. See below for images.
First off, quarantine the infected plant. Take it to a room where it is still getting similar light but isn't around other plants that the pests can spread to.
Dilute a very gentle soap like Dawn or Dr. Bronner's castile soap by adding a couple of drops to a mason jar of water. Give it a good shake and dip a clean, soft rag into the water before using to wipe off any visible pests, rinsing the rag out in between passes. Another approach for first line of defense on hard-bodied pests like scale or mealybug is to dip Q-tips into rubbing alcohol and dab the areas where you see the pests hiding, removing them and killing any remaining nymphs in the process.
Follow up removal with treatment. Using a spray bottle of properly diluted neem oil, horticultural oil or insecticidal soap, thoroughly spray the top sides and undersides of each leaf, all along the stem and along the top of the soil.
If things have progressed and you have a real infestation on your hands or you're battling hard-bodied pests like scale, bring in the big guns with a systemic. This class of pesticides gets its name from the fact that the plant absorbs it through its roots and as the pests feed on the leaves and stems, they ingest the pesticide into their system. Systemics are sold as granules that are sprinkled atop the soil and penetrate the roots when watered. Be sure and follow the instructions and do not overapply. Generally, a little will go a long way.
Repeat steps once every 7-10 days for a few weeks, and continue for a week or two after you're no longer seeing the pest, just to be proactive.
TIPS FOR SPECIFIC PESTS
Spider mites show up mostly in winter when the air is dry. They're detectable by their webbing and look like teeny-tiny brown flecks within the white webs. Keep plants well hydrated to avoid them, and if they do settle in, try using a sprayer faucet to blast them off the plant with water then follow up with a mite killer like Safer's 3-in-1.
Fungus gnats are the polar opposite of spider mites, thriving in moist conditions. Make sure to keep fallen leaves or debris off the soil surface, and let the plant dry out about three inches down before watering. If that doesn't do the trick, you can replace the top few inches of soil, add a sticky stake to the pot and/or go for a systemic.
Scale look like small reddish-brown bumps and generally take up residence on the undersides of leaves. You can scrape them off with a toothbrush or fingernail but should follow up with a systemic.
Mealybugs look like fuzzy white tufts of cotton, and are typically found at the joints where the leaves meet the stem. If they're covering even half of your plant, just give up and move. Okay, that might be drastic, but the reality is that mealybugs are a nightmare to get rid of so you need to weigh how much you have invested in that plant before settling in to fight them. If you're up for it, follow all steps outlined above, and keep at it.
Aphids are tiny oval guys also called plant lice and can be black, red, green or yellow, but most commonly green. They will typically be found on new growth, the tips of plants where new leaves are about to unfurl. A strong spray of water from a sprayer faucet should knock most of them off, then follow up with applying insecticidal soap.