How to let the app know if it’s running Unit tests in a pure Swift project?

Elvind’s answer isn’t bad if you want to have what used to be called pure “Logic Tests”. If you’d still like to run your containing host application yet conditionally execute or not execute code depending on whether tests are run, you can use the following to detect if a test bundle has been injected:

if NSProcessInfo.processInfo().environment["XCTestConfigurationFilePath"] != nil {
     // Code only executes when tests are running
}

I used a conditional compilation flag as described in this answer so that the runtime cost is only incurred in debug builds:

#if DEBUG
    if NSProcessInfo.processInfo().environment["XCTestConfigurationFilePath"] != nil {
        // Code only executes when tests are running
    }
#endif

Edit Swift 3.0

if ProcessInfo.processInfo.environment["XCTestConfigurationFilePath"] != nil {
    // Code only executes when tests are running
}