How can I tell if an Android app supports Wear, before installing it? Is there a way to tell if an Android app has Wear support, before installing the app on a phone paired with a Wear device?

Shazam for example supports Wear, but there's no metadata (apparently) in its Play listing indicating Wear support. Yes, the description says "Wear", but that is up to the developer, and may be missing - Foursquare doesn't have "Wear" in its description, but they did publish a Wear app.
Not sure on the web side when viewing from a computer, viewing the app in the Play Store on your mobile device it'll show a watch icon in the Description area right above "Read More".  If you hit "Read More" and scroll all the way down, if it is "Enhanced for Android Wear", you'll see the same watch icon and information about Android Wear.