You can see, with most products, if they've been used. Look sideways in the light, at the top of a lip balm or lipsticks, across the surface of a blush or eyeshadow, and at the lid of any lotions in a bottle or jar. You are really can tell if something has been even the slightest bit sampled.
For some products, with antioxidants in them, you will want to make sure they have actually never been OPENED, with is different than being used. Much of the good stuff that you are paying for degrades quickly upon exposure to air. For things like lotions, the best ones are those in opaque containers that do not let air into the product when opened.
Of course, the best and most safe packaging usually adds extra environmental waste to the planet, so it's a tough call.
My call is that for things going near my eyes and lips - the mucous membranes - to be ultra careful nothing has been *used*.
For things that I pay good money for to be state of the art, as in a moisturizer with peptides, antioxidants and cell-mimicking ingredients, i make sure it's never been *opened.*