When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
A date code is molded into the plastic case. All you can do is check that. The battery is sealed inside the sensor. Look for what looks like a small clock face, that is molded in the case. The arrow points to the month. On either side of the arrow will be a number indicating the year of manufacture. EX. arrow points to 10, and 9 8 are in the center on either side of the arrow= Oct, 1998 mfg date.
If it's at the 7+ year mark, it's probably not going to last much longer unless you have a car with low milage, where the battery has been unused for most of it's life. If your the DIY type, there is a thread with pictures, that shows what's involved in replacing the battery. It's in the archives.