At the trial you spend a good while sitting in front of the kit, so you can watch calmly.
Of those four things, the easiest to see are the sponges, the powder puffs and the brushes: a sponge or a powder puff has to be new, and a brush has to be cleaned between clients, with no traces of another person's makeup. A glance is enough.
If you have any doubts, I always encourage you to ask. How the kit is cleaned is as normal a question as which brands I use. Never keep a doubt to yourself: in the end it is your skin, and you need to feel sure. I explain everything I use and how I clean it in the article on makeup artist sanitation.
