How to determinate Apple Watch model at runtime.

watchType.swift

import Foundation
import UIKit // Only to use CGFloat 😬
import WatchKit

enum WatchModelType {
    case large
    case small
}

struct WatchModel {
    static let largeWith: CGFloat = 156
    static let largeHeight: CGFloat = 195
    
    /// Returns the model of the Watch
    ///
    /// - Returns: the model of the Watch as `WatchModel`
    static func model() -> WatchModelType {
        // Watch 42mm bounds: (0.0, 0.0, 156.0, 195.0)
        // Watch 38mm bounds: (0.0, 0.0, 136.0, 170.0)
        let bounds = WKInterfaceDevice.current().screenBounds
        switch (bounds.width, bounds.height) {
        case (largeWith, largeHeight):
            return .large
        default:
            return .small
        }
    }
}