Open your case and check that the o-ring has a slight indentation all the way around it. This is caused by the compression of the lid when the case is shut. In short, if the indentation is there, the seal is working. If you want total piece of mind, proceed to step 2...
2. Water test
If you want to be extra sure, place a wad of tissue paper in the case, shut the lid and lock down the latches. Put the case in the sink, bath or shower and run the water directly onto it from multiple angles for five minutes. Inspect the tissue paper afterwards for any signs of moisture.
3. Our warranty
99.9% of cases will pass these tests with ease, but as with any products, there is a chance of manufacturing defects. If you're concerned, call or email us immediately. Our warranty is there to make sure our customers get nothing less than the highest standard of product and service.