I would not worry about the whole sand thing, unless you live in a very dry sandy area, you have your horses out on a dirt lot, or you have a horse that eats dirt you most likely have nothing to worry about. (also keep a large supply of clean water and it will bring those chance down even more - this is a given just saying because there are some silly people out there!)
Someone may have already told you how to test for the amount of sand your horse may consume above but I did not read through all of the post.
Smartpak (a supplement company) has a very good how to way to test you horses poop, they also have a wide variety of supplements for your horses needs.
As far as having/getting a mineral and/or salt block for your horse I say better safe then sorry.
I keep both out all year long.
Like I have said in another thread, I like to believe the feed and hay I offer my horses provides them with everything they need but if not, and believe me it makes a difference it your horses health if they don't, I like to think I am providing them with anything extra that they may need.
A horse knows what is missing in it diet and will turn to other things to "feel those gaps". So I feel leaving them free choice minerals and salt will allow them to get those "extras" from a safe place.
P.s. I tried to find the How To that smartpak had up on their website at one time but I am finding it hard to find. I will keep looking, and I know I have it in a book at home, when I find it I will send you a link.