Without testing there is really no way to tell. Generally, silica sand is white, but there are other white minerals other than silica, and depending where the sand was taken from, the composition can vary from place to place. There is the acid test, where a sample of the sand is placed in a test tube and covered with white vinegar or stronger acid. I have suggested before that probably any pharmacist would be willing to make the test, using a stronger acid which would give very visable results quickly. With vinegar, you have to closely observe the sand for any signs of fizz, which would indicate carbonates or other detrimental minerals.
After becoming familiar with silica sand it isn't hard to recognize it. The finer grade of sand is less optimal than the more coarse forms, but it will work 50/50 with peat, more or less. It tends to compact though. If this grade is all I have to work with, then I use maybe 1/4 part LFS to break things up a little more and prevent the mix from packing down.
Have you tried swimming supply stores for silica sand? Pool filter sand is generally of the coarser type, and so far all I have seen has been silica. That's not to say there couldn't be other types in other areas though. You see what I mean? You shouldn't assume any given batch of sand is the same as any other.
As a rule of thumb, I try to avoid sand with multi colored grains. This is the sort that looks superficially beige or tan colored. I trust only white sand, and even then if I have any doubts that is is silica I do the fizz test.
Other sand to avoid is river sand, or sand that looks overly dusty or dirty. Whatever you use, be sure to rinse it well before use.