If you can see the returns in the pool, you can test the filter. Buy a small box of DE at a pool store (don't let 'em sell you anything else) and add a half Cup to a Cup to the skimmer and watch the returns. If DE comes out, there's a problem with the filter. Watch the filter pressure, a sand filter with DE will build pressure quickly. Take a look at this thread: http://pool9.net/de-test/, it discusses using DE in a sand filter.