The best thing to do is give it to random friends or family and have them play it, and observe. Let people who typically don't play videogames too much the game to try out. Observe challenging sections that everyone gets stuck on and make them less challenging. Also constant checkpoints and infinite lives can make challenging games less stressful.
Well you can try to make is as hard as you want and then give your players an ability to choose other difficulty level (like normal and easy) and give them some starting bonuses based on that choice
