I'm looking to buy a smartwatch. How can I tell if an app in the Play Store is compatible with it before buying the device? I want to buy a smartwatch, but I am trying to research whether apps will will work with it before buying. Is there any way to do that?
The easiest way to check is to make sure it is labeled either "Wear OS", "Wear OS by Google" or "Android Wear". 

Another way to check is if the watch supports the Google Assistant, it most likely is running Wear OS and should be supported as well.

Finally, if you want to be absolutely positive, you can always check the Wear OS website