Step 1:Press ESC.
Step 2:Press Open to LAN
Step 3:Leave The Gamemode anything and Allow Cheats On
Step 4:Say Yes,Confirm
Step 5:Do a Random Command like /gamemode 1 /kill
Optional:If you dont know any other commands type /help /help 2 /help 3 /help 4
Ways to see if the World dosent have cheats on
If your creating once go to more settings and look at Allow Cheats:On/Off
Go at Menu Click Single Player look at your saved world look below the world's Title and see if theres Cheats On
If theres none then there is no cheats in it