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
}

Instead of checking if the tests are running to avoid side-effects, you could run the tests without the host app itself. Go to Project Settings -> select the test target -> General -> Testing -> Host Application -> select 'None'. Just remember to include all files you need to run the tests, as well as libraries normally included by the Host app target.

I use this in application:didFinishLaunchingWithOptions:

// Return if this is a unit test
if let _ = NSClassFromString("XCTest") {
    return true
}