How to tell at runtime whether an iOS app is running through a TestFlight Beta install

Based on combinatorial's answer I created the following SWIFT helper class. With this class you can determine if it's a debug, testflight or appstore build.

enum AppConfiguration {
  case Debug
  case TestFlight
  case AppStore
}

struct Config {
  // This is private because the use of 'appConfiguration' is preferred.
  private static let isTestFlight = Bundle.main.appStoreReceiptURL?.lastPathComponent == "sandboxReceipt"
  
  // This can be used to add debug statements.
  static var isDebug: Bool {
    #if DEBUG
      return true
    #else
      return false
    #endif
  }

  static var appConfiguration: AppConfiguration {
    if isDebug {
      return .Debug
    } else if isTestFlight {
      return .TestFlight
    } else {
      return .AppStore
    }
  }
}

We use these methods in our project to supply different tracking id's or connection string per environment:

  func getURL(path: String) -> String {    
    switch (Config.appConfiguration) {
    case .Debug:
      return host + "://" + debugBaseUrl + path
    default:
      return host + "://" + baseUrl + path
    }
  }

OR:

  static var trackingKey: String {
    switch (Config.appConfiguration) {
    case .Debug:
      return debugKey
    case .TestFlight:
      return testflightKey
    default:
      return appstoreKey
    }
  }

UPDATE 05-02-2016: A prerequisite to use a preprocessor macro like #if DEBUG is to set some Swift Compiler Custom Flags. More information in this answer: https://stackoverflow.com/a/24112024/639227

Update 23-01-2024: Note that this does not work for Mac catalyst app, their receipt url is /Applications/<app>/Contents/_MASReceipt/receipt even on TestFlight.

Modern Swift version, which accounts for Simulators (based on accepted answer):

var isSimulatorOrTestFlight: Bool {
    guard let path = Bundle.main.appStoreReceiptURL?.path else {
        return false
    }
    return path.contains("CoreSimulator") || path.contains("sandboxReceipt")
}

Update 23-01-2024: Note that this does not work for Mac catalyst app, their receipt url is /Applications/<app>/Contents/_MASReceipt/receipt even on TestFlight.

I use extension Bundle+isProduction on Swift 5.2:

import Foundation

extension Bundle {
    var isProduction: Bool {
        #if DEBUG
            return false
        #else
            guard let path = self.appStoreReceiptURL?.path else {
                return true
            }
            return !path.contains("sandboxReceipt")
        #endif
    }
}

Then:

if Bundle.main.isProduction {
    // do something
}