Swift 4 implementation for checking if segue with identifier exists.

CanPerformSegue.swift

import UIKit

extension UIViewController {
  
 /**
  Checks whether controller can perform specific segue or not.
  - parameter identifier: Identifier of UIStoryboardSegue.
  */
 func canPerformSegue(withIdentifier identifier: String) -> Bool {
   //first fetch segue templates set in storyboard.
   guard let identifiers = value(forKey: "storyboardSegueTemplates") as? [NSObject] else {
     //if cannot fetch, return false
      return false
   }
   //check every object in segue templates, if it has a value for key _identifier equals your identifier.
    let canPerform = identifiers.contains { (object) -> Bool in
      if let id = object.value(forKey: "_identifier") as? String {
        if id == identifier{
          return true
        } else {
          return false
        }
      } else {
        return false
      }
    }
    return canPerform
  }
}