airtag battery level

you can't, you can only get a notification when it's running low

As Rudegar has already mentioned, Apple does not provide you with a method to check the battery life on their AirTags. You will only get a notice in the Find My app that the battery level is low.

For reference, Apple says that the typical battery life is around 1 year. Like all batteries, temperature extremes can shorten that time span.