You need to apply the water pressure behind the sample till 3 drops of the water can pass through it. You can pour the water on the fabric as well and wait for some time. Make sure that there is no wet stain on the other side of the tent. Usually, it is simple to notice if your tent leaks. I also recommend spending a night in the tent in your yard during the rain before going on a trip, if you have free time to do this.