It depends what you mean by 'touch'. I have a long hair system with is reasonable thick (but not ridiculously thick). If by 'touch' you mean someone just puts their hand on it and casually touches the hair, then in my view I VERY much doubt they could detect any kind of hair system on my current one. Mine just feels like normal hair, and when I touch it myself I can't even feel the base as it's so thin and featherlight (Swiss lace).
However, if by 'touch' you mean vigorously running their fingers though your hair then I cannot guarantee they wouldn't notice SOMETHING, perhaps an edge etc. Whether they'd know it's a hair system I don't know. Most people don't know about hair systems anyway.
This is one advantage of thicker or longer hair systems in my opinion - they are less likely to be detected, especially to touch because there is more hair to cover the edges, and more hair covering the base. For example, if I tie my hair back as opposed to letting it down, then edges kind of 'lock' the hair system in place, so the edges can't be seen or felt at all. As long as a thicker hair system doesn't LOOK ridiculously obvious, then there are many advantages to them I think. You can do more with them in terms of how you style them, and they also last longer because there's more hair in the system so even with some shedding it won't matter as much as it would with a thinner density system.