All you need Skyzoo is a handfull of sand and whilst holding it in your hand put your hand in a bucket of water and see how much if any silt comes out of it, my guess is that there won't be any silt coming out if it is truly sea sand. I go to the pit and test any sand I'm thinking of using for the first time like this, tried some last week that by the time the silt had washed out into the water there wasn't hardly a thing left in my hand, this sand (mud) would have suffered from terrible shrinkage, the pit owner was really upset when I said his sand wasn't even fit to fill trenches.