IP rating is for ingress protection against water. For example, IP68 rating protects against dust and water for up to 1.5meters depth for 30 minutes. Whereas, ATM means the atmospheric pressure your smartwatch can bear at a certain depth/ A smartwatch with 5ATM rating can withstand up to 50 meter deep water.
The easiest way to find the IP rating on your smartwatch is to check the manufacturer's website. Head to the smartwatch product page on brand's official website, and check for water resistance rating under specifications.
Alternatively, go to a reliable e-commerce platform and check the listing page for your smartwatch if available. Any water resistance or protection is usually mentioned under specifications.
Furthermore, you can check the rear of your smartwatch box or user manual for the mention of water resistance if any. Most brands usually write it on the features section on the box.
Most modern smartwatches come with IP68 water-resistant rating. However, some of them pack both IP68 and 5ATM protection which allows the watch to be used while swimming. In general, it's always better to have both IP and ATM rating for an extra peace of mind when exposing the smartwatch to water.
What's more? - Significance of IP and ATM rating - IP68 vs 5ATM Rating - Tips to Follow for Waterproof Watches -What to Do if Your Smartwatch Gets Wet?