Long before the first frost hits, TV ads nag you incessantly about the condition of your antifreeze. "Will it protect you through the winter ahead?" they ask. "In fact, shouldn't you be checking it right now, instead of watching football?" Checking the potency won't stop the commercials, but it will let you enjoy the game guilt-free.

You can check antifreeze in one of three ways, from high- to highest-tech:

• Test strips. Available in packets at most auto supply stores for a few bucks, antifreeze test strips work just like litmus paper. Dunk the tip in your radiator, wait a minute, and then compare the colors with those on an accompanying reference chart.

Some test strips have the unique advantage of showing not only the antifreeze's concentration (which determines its freezing point) but also its alkalinity (which indicates its corrosion protection). In fact, corrosion protection deteriorates much faster than freeze-up protection. But because different antifreeze brands have different optimal alkalinities, you need to know the exact kind in your car to determine the state of its corrosion protection. And if you've mixed two different kinds, then all bets are off. That's why many experts recommend relying on a schedule for antifreeze changes rather than on an alkalinity test.

Furthermore, test strips require a degree of care and a good eye for color to read accurately. For instance, the dyes used in some long-life antifreeze brands can affect the color of the strip and make an exact match to the reference chart tricky. Even the weather may affect the window of time you have to take a good reading; the strip changes color as it dries, and the color changes more slowly in cooler weather.

• Hydrometer. For about as much as you'd pay for a packet of test strips, you can buy a hydrometer that will last a lifetime. It looks like a big turkey baster and checks the potency of the antifreeze by measuring its specific gravity.

Using a squeezable bulb, you suck up a little antifreeze into a clear chamber, where a scale tells you the freezing point. Hydrometers with a calibrated float inside work better than ones with a simple ball float.

Unlike test strips, however, a hydrometer will tell you only the freezing point. It won't measure the corrosion protection. Also, if your car has a less-toxic propylene glycol-based antifreeze formula (rather than a conventional or long-life ethylene glycol-based formula), you need a special hydrometer or special test strips.

• Refractometer. Nowadays, most repair shops use a pricey device called a refractometer, which checks the antifreeze concentration by measuring the degree to which light bends as it passes through the mixture. It gives very fast, accurate readings and works with any type of antifreeze. A good shop will probably check the antifreeze for free while you're having other work done.

Once you've determined the concentration, add antifreeze and water as necessary to get a 50/50 mix, which should protect against freezing down to -34 degrees. But do so carefully; too-high concentrations of antifreeze may actually raise the freezing point of the mixture.