How do I check AirTag battery before placing in luggage?

Unfortunately, Apple hasn't provided us with any simiple method to check the AirTag's battery level other than when it is about to be fully depleted.

That leave you with a couple of options:

• If the AirTag is older than 8 months old, you may want to consider replacing it. Apple claims that the typical battery life, under normal use, is approx. 1 year.
• Temporarily remove the battery, and then, use a voltmeter / battery tester to check the current battery voltage. However, this doesn't really tell you when the battery will become depleted. Here lithium batteries tend to maintain a steady voltage until the end of life, so it could still be a good indicator of battery health.