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.
Hrm.... I might have screwed up. LOL. But my plants are seemingly happy though... Just made my mix and washed it thoroughly. Plants are already appearing to be coming out of shock after couple days and my sundews are getting dew. Maybe I was just lucky?
I never want to appear as an alarmist, lol. Truth is, I really can't say what your sand is like, but if the plants are doing ok, that is a good sign in and of itself - but it is not the end of the story. Over time salts leach out of non-silica sand, and concentrate in the mix over time. If you notice a distinct decline in your older plants in this mix, this is something to consider. Since I have ready access to good quality sand, I just go with that and shun all other types. The only to know if any sand is ok is to make the test yourself and grow some plants in it. Even with the "iffieness" in the sand issue, I still prefer it over perlite in most instances of Drosera cultivation which is my main focus.