Pfthb

Project build settings have a block 'Swift Compiler - Languages', which stores information about Swift Language Version in key-value format. It will show you all available (supported) Swift Language Version for your Xcode and active version also by a tick mark.

Project ► (Select Your Project Target) ► Build Settings ► (Type
'swift_version' in the Search bar) Swift Compiler Language ► Swift Language
Version ► Click on Language list to open it (and there will be a tick mark on any one of list-item, that will be current swift version).

Look at this snapshot, for easy understanding:

With help of following code, programmatically you can find Swift version supported by your project.

#if swift(>=5.0)
 print("Hello, Swift 5.0")

#elseif swift(>=4.2)
 print("Hello, Swift 4.2")

#elseif swift(>=4.1)
 print("Hello, Swift 4.1")

#elseif swift(>=4.0)
 print("Hello, Swift 4.0")

#elseif swift(>=3.2)
 print("Hello, Swift 3.2")

#elseif swift(>=3.0)
 print("Hello, Swift 3.0")

#elseif swift(>=2.2)
 print("Hello, Swift 2.2")

#elseif swift(>=2.1)
 print("Hello, Swift 2.1")

#elseif swift(>=2.0)
 print("Hello, Swift 2.0")

#elseif swift(>=1.2)
 print("Hello, Swift 1.2")

#elseif swift(>=1.1)
 print("Hello, Swift 1.1")

#elseif swift(>=1.0)
 print("Hello, Swift 1.0")

#endif

Here is result using Playground (with Xcode 10.x)

What I do is say in the Terminal:

$ xcrun swift -version

Output for Xcode 6.3.2 is:

Apple Swift version 1.2 (swiftlang-602.0.53.1 clang-602.0.53)

Of course that assumes that your xcrun is pointing at your copy of Xcode correctly. If, like me, you're juggling several versions of Xcode, that can be a worry! To make sure that it is, say

$ xcrun --find swift

and look at the path to Xcode that it shows you. For example:

/Applications/Xcode.app/...

If that's your Xcode, then the output from -version is accurate. If you need to repoint xcrun, use the Command Line Tools pop-up menu in Xcode's Locations preference pane.

Open the Terminal and write:

swift -version

From Xcode 8.3 onward Build Settings has key Swift Language Version with a value of swift version your target is using.

For older Xcodes use this solution,
open terminal and type following command(s)

Case 1: You have installed only one Xcode App

swift -version

Case 2: You have installed multiple Xcode Apps

• 
  

  Switch active developer directory (Replace Xcode_7.3.app from
  following command with your Xcode app file name from Application directory for which you want
  to check swift version)

  
  
  

   sudo xcode-select --switch /Applications/Xcode_7.3.app/Contents/Developer
  

  
  


• 
  

  Then

  
  
  

   swift -version
  

  
  

NOTE: From Xcode 8 to Xcode 8.2.x you can use swift 2.3 even though Xcode 8 uses swift 3.x as default swift version. To use swift 2.3, just turn on flag Use Legacy Swift Language Version to YES from Build Setting and XCode will use Swift 2.3 for that project target.

This reddit post helped me:
https://www.reddit.com/r/swift/comments/4o8atc/xcode_8_which_swift/d4anpet

Xcode 8 uses Swift 3.0 as default. But you can turn on Swift 2.3. Go to project's Build Settings and set 'Use Legacy Swift Language Version' to YES.

Good old reddit :)