Those consruction workers need to memorize this ditty:
Coral snakes vs King snakes:

Red touch black, poison lack
Red touch yellow, kill a fellow.

If you can get close enough, you can look at the eyes. Non-poisonous snakes have round pupils like we do, but poisonous ones have vertical slits like that most evil of all creatures, the cat.

Your wild caught fellow is one lucky snake that you came along and befriended him. Good on you!