What you're probably noticing
Maybe the hot water doesn't last as long as it used to. A shower that used to run fifteen minutes now turns lukewarm at ten. Or you've caught a faint rotten egg smell near the utility closet. Or you just did the math and realized the tank is pushing eight, nine years old and you're wondering if it's going to pick the worst possible week in January to quit on you. All of these point back to the same small part most people have never heard of: the anode rod.
The part doing all the work
Every standard tank water heater has a long metal rod bolted into the top, usually made of magnesium or aluminum wrapped around a steel wire core. It costs very little compared to the tank itself. Its entire job is to corrode first, so the steel tank doesn't have to. Water, especially the hard, mineral heavy water a lot of Nixa and Christian County wells and municipal lines deliver, is naturally corrosive to bare metal. The anode rod sacrifices itself to that chemistry so the tank lining stays intact longer.
Once the rod is eaten down to the wire core, it stops protecting anything. From that point on, the tank itself starts corroding. That's when you get pinhole leaks, rust colored water, or a slow failure that ends with water on the garage or basement floor.
More on this from The Cheap Part That Decides How Long A Tank Lasts.
Why this matters more here than you'd think
Southwest Missouri water tends to run hard, and well water around Nixa and the surrounding unincorporated parts of Christian County can be harder still, depending on the source. Hard water accelerates anode rod wear. A rod that might last five or six years in soft water conditions can be used up in three or four here. Most people never check it because nothing on the outside of the tank tells you it's happening.
The other local factor is temperature swings. Ozark summers push tank thermostats and relief valves harder as groundwater coming into the house warms up and usage patterns change with kids home and more showers. Winter cold snaps do the opposite, making the tank work harder to recover between uses. Neither of these wear out the anode rod directly, but they do shorten the life of a tank that's already lost its corrosion protection, so problems tend to surface at the seasonal extremes rather than in a mild stretch of October.
What you can actually check yourself
If your water heater has an easily accessible hex head on top, under the outer jacket, some homeowners do pull the rod themselves with a long ratchet and a breaker bar. If it comes out looking like a thin wire with little or no metal left around it, that's your answer. A healthy rod still has a solid half inch to an inch of material around the core.
Short of pulling it, there are signs you can watch for without any tools. A sulfur or rotten egg smell in hot water only, not cold, often means bacteria reacting with a worn down magnesium rod. Rust colored hot water, even if it clears after running for a minute, is worth paying attention to. A tank older than six or seven years that's never had any attention to the rod is playing the odds.
Where it stops being a DIY job
Pulling the rod sounds simple until the fitting won't budge. On older tanks, especially ones that have sat in a Nixa garage or crawlspace through a few summers and winters, that hex head corrodes itself into place. People have sheared bolts, cracked the tank's top plate, or stripped the fitting trying to force it with a standard ratchet, which turns a cheap part replacement into a full tank replacement.
If the rod won't come out with reasonable hand pressure, if you're not sure which type of rod your tank needs, or if you've already got rust colored water and a tank past the ten year mark, that's the point to call someone who does water heater repair regularly. It's also worth having a professional take a look if you're not confident shutting off and draining the tank safely first, since skipping that step is how people get scalded or flood a utility closet. A rod swap is inexpensive and quick for someone who does it often. A tank that fails because the rod was ignored too long is not.